Support identical cron schedule (#87)

* Retry on lock being unavailable

* Refactor locking to return plain error

* Collect LockedTime in stats

* Add test case

* Add documentation for LOCK_TIMEOUT

* Log in case lock needs to be awaited

* Release resources created for awaiting lock

cmd/backup/main.go

@@ -8,14 +8,15 @@ import (
 )
 
 func main() {
-	unlock := lock("/var/lock/dockervolumebackup.lock")
-	defer unlock()
-
 	s, err := newScript()
 	if err != nil {
 		panic(err)
 	}
 
+	unlock, err := s.lock("/var/lock/dockervolumebackup.lock")
+	defer unlock()
+	s.must(err)
+
 	defer func() {
 		if pArg := recover(); pArg != nil {
 			if err, ok := pArg.(error); ok {