Rewrite queue (#24505)

# ⚠️ Breaking

Many deprecated queue config options are removed (actually, they should
have been removed in 1.18/1.19).

If you see the fatal message when starting Gitea: "Please update your
app.ini to remove deprecated config options", please follow the error
messages to remove these options from your app.ini.

Example:

```
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].ISSUE_INDEXER_QUEUE_TYPE`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].UPDATE_BUFFER_LEN`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [F] Please update your app.ini to remove deprecated config options
```

Many options in `[queue]` are are dropped, including:
`WRAP_IF_NECESSARY`, `MAX_ATTEMPTS`, `TIMEOUT`, `WORKERS`,
`BLOCK_TIMEOUT`, `BOOST_TIMEOUT`, `BOOST_WORKERS`, they can be removed
from app.ini.

# The problem

The old queue package has some legacy problems:

* complexity: I doubt few people could tell how it works.
* maintainability: Too many channels and mutex/cond are mixed together,
too many different structs/interfaces depends each other.
* stability: due to the complexity & maintainability, sometimes there
are strange bugs and difficult to debug, and some code doesn't have test
(indeed some code is difficult to test because a lot of things are mixed
together).
* general applicability: although it is called "queue", its behavior is
not a well-known queue.
* scalability: it doesn't seem easy to make it work with a cluster
without breaking its behaviors.

It came from some very old code to "avoid breaking", however, its
technical debt is too heavy now. It's a good time to introduce a better
"queue" package.

# The new queue package

It keeps using old config and concept as much as possible.

* It only contains two major kinds of concepts:
    * The "base queue": channel, levelqueue, redis
* They have the same abstraction, the same interface, and they are
tested by the same testing code.
* The "WokerPoolQueue", it uses the "base queue" to provide "worker
pool" function, calls the "handler" to process the data in the base
queue.
* The new code doesn't do "PushBack"
* Think about a queue with many workers, the "PushBack" can't guarantee
the order for re-queued unhandled items, so in new code it just does
"normal push"
* The new code doesn't do "pause/resume"
* The "pause/resume" was designed to handle some handler's failure: eg:
document indexer (elasticsearch) is down
* If a queue is paused for long time, either the producers blocks or the
new items are dropped.
* The new code doesn't do such "pause/resume" trick, it's not a common
queue's behavior and it doesn't help much.
* If there are unhandled items, the "push" function just blocks for a
few seconds and then re-queue them and retry.
* The new code doesn't do "worker booster"
* Gitea's queue's handlers are light functions, the cost is only the
go-routine, so it doesn't make sense to "boost" them.
* The new code only use "max worker number" to limit the concurrent
workers.
* The new "Push" never blocks forever
* Instead of creating more and more blocking goroutines, return an error
is more friendly to the server and to the end user.

There are more details in code comments: eg: the "Flush" problem, the
strange "code.index" hanging problem, the "immediate" queue problem.

Almost ready for review.

TODO:

* [x] add some necessary comments during review
* [x] add some more tests if necessary
* [x] update documents and config options
* [x] test max worker / active worker
* [x] re-run the CI tasks to see whether any test is flaky
* [x] improve the `handleOldLengthConfiguration` to provide more
friendly messages
* [x] fine tune default config values (eg: length?)

## Code coverage:

![image](https://user-images.githubusercontent.com/2114189/236620635-55576955-f95d-4810-b12f-879026a3afdf.png)

modules/queue/base_levelqueue_common.go

@@ -0,0 +1,92 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package queue
+
+import (
+	"context"
+	"fmt"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"code.gitea.io/gitea/modules/nosql"
+
+	"gitea.com/lunny/levelqueue"
+	"github.com/syndtr/goleveldb/leveldb"
+)
+
+type baseLevelQueuePushPoper interface {
+	RPush(data []byte) error
+	LPop() ([]byte, error)
+	Len() int64
+}
+
+type baseLevelQueueCommonImpl struct {
+	length   int
+	internal baseLevelQueuePushPoper
+	mu       *sync.Mutex
+}
+
+func (q *baseLevelQueueCommonImpl) PushItem(ctx context.Context, data []byte) error {
+	return backoffErr(ctx, backoffBegin, backoffUpper, time.After(pushBlockTime), func() (retry bool, err error) {
+		if q.mu != nil {
+			q.mu.Lock()
+			defer q.mu.Unlock()
+		}
+
+		cnt := int(q.internal.Len())
+		if cnt >= q.length {
+			return true, nil
+		}
+		retry, err = false, q.internal.RPush(data)
+		if err == levelqueue.ErrAlreadyInQueue {
+			err = ErrAlreadyInQueue
+		}
+		return retry, err
+	})
+}
+
+func (q *baseLevelQueueCommonImpl) PopItem(ctx context.Context) ([]byte, error) {
+	return backoffRetErr(ctx, backoffBegin, backoffUpper, infiniteTimerC, func() (retry bool, data []byte, err error) {
+		if q.mu != nil {
+			q.mu.Lock()
+			defer q.mu.Unlock()
+		}
+
+		data, err = q.internal.LPop()
+		if err == levelqueue.ErrNotFound {
+			return true, nil, nil
+		}
+		if err != nil {
+			return false, nil, err
+		}
+		return false, data, nil
+	})
+}
+
+func baseLevelQueueCommon(cfg *BaseConfig, internal baseLevelQueuePushPoper, mu *sync.Mutex) *baseLevelQueueCommonImpl {
+	return &baseLevelQueueCommonImpl{length: cfg.Length, internal: internal}
+}
+
+func prepareLevelDB(cfg *BaseConfig) (conn string, db *leveldb.DB, err error) {
+	if cfg.ConnStr == "" { // use data dir as conn str
+		if !filepath.IsAbs(cfg.DataFullDir) {
+			return "", nil, fmt.Errorf("invalid leveldb data dir (not absolute): %q", cfg.DataFullDir)
+		}
+		conn = cfg.DataFullDir
+	} else {
+		if !strings.HasPrefix(cfg.ConnStr, "leveldb://") {
+			return "", nil, fmt.Errorf("invalid leveldb connection string: %q", cfg.ConnStr)
+		}
+		conn = cfg.ConnStr
+	}
+	for i := 0; i < 10; i++ {
+		if db, err = nosql.GetManager().GetLevelDB(conn); err == nil {
+			break
+		}
+		time.Sleep(1 * time.Second)
+	}
+	return conn, db, err
+}