Multiple Queue improvements: LevelDB Wait on empty, shutdown empty shadow level queue, reduce goroutines etc (#15693)

* move shutdownfns, terminatefns and hammerfns out of separate goroutines

Coalesce the shutdownfns etc into a list of functions that get run at shutdown
rather then have them run at goroutines blocked on selects.

This may help reduce the background select/poll load in certain
configurations.

* The LevelDB queues can actually wait on empty instead of polling

Slight refactor to cause leveldb queues to wait on empty instead of polling.

* Shutdown the shadow level queue once it is empty

* Remove bytefifo additional goroutine for readToChan as it can just be run in run

* Remove additional removeWorkers goroutine for workers

* Simplify the AtShutdown and AtTerminate functions and add Channel Flusher

* Add shutdown flusher to CUQ

* move persistable channel shutdown stuff to Shutdown Fn

* Ensure that UPCQ has the correct config

* handle shutdown during the flushing

* reduce risk of race between zeroBoost and addWorkers

* prevent double shutdown

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/indexer/code/indexer.go

@@ -115,7 +115,13 @@ func Init() {
 
 	ctx, cancel := context.WithCancel(context.Background())
 
-	graceful.GetManager().RunAtTerminate(ctx, func() {
+	graceful.GetManager().RunAtTerminate(func() {
+		select {
+		case <-ctx.Done():
+			return
+		default:
+		}
+		cancel()
 		log.Debug("Closing repository indexer")
 		indexer.Close()
 		log.Info("PID: %d Repository Indexer closed", os.Getpid())