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/script.go

@@ -47,6 +47,8 @@ type script struct {
 	file  string
 	stats *Stats
 
+	encounteredLock bool
+
 	c *Config
 }