Restore Graceful Restarting & Socket Activation (#7274)

* Prevent deadlock in indexer initialisation during graceful restart

* Move from gracehttp to our own service to add graceful ssh

* Add timeout for start of indexers and make hammer time configurable

* Fix issue with re-initialization in indexer during tests

* move the code to detect use of closed to graceful

* Handle logs gracefully - add a pid suffix just before restart

* Move to using a cond and a holder for indexers

* use time.Since

* Add some comments and attribution

* update modules.txt

* Use zero to disable timeout

* Move RestartProcess to its own file

* Add cleanup routine

modules/ssh/ssh_windows.go

@@ -0,0 +1,24 @@
+// +build windows
+
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"code.gitea.io/gitea/modules/log"
+	"github.com/gliderlabs/ssh"
+)
+
+func listen(server *ssh.Server) {
+	err := server.ListenAndServe()
+	if err != nil {
+		log.Critical("Failed to serve with builtin SSH server. %s", err)
+	}
+}
+
+// Unused does nothing on windows
+func Unused() {
+	// Do nothing
+}