Prevent timer leaks in Workerpool and others (#11333)

There is a potential memory leak in `Workerpool` due to the intricacies of
`time.Timer` stopping.

Whenever a `time.Timer` is `Stop`ped its channel must be cleared using a
`select` if the result of the `Stop()` is `false`.

Unfortunately in `Workerpool` these were checked the wrong way round.

However, there were a few other places that were not being checked.

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

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/util/timer.go

@@ -0,0 +1,21 @@
+// Copyright 2020 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 util
+
+import (
+	"time"
+)
+
+// StopTimer is a utility function to safely stop a time.Timer and clean its channel
+func StopTimer(t *time.Timer) bool {
+	stopped := t.Stop()
+	if !stopped {
+		select {
+		case <-t.C:
+		default:
+		}
+	}
+	return stopped
+}