Generalize watcher interface

[oklahomer]

Currently, an interface called watchers.Watcher and its implementation is provided to subscribe to the changes of files under a predefined directory. When a Command or ScheduledTask's configuration file under the directory is updated, this implementation calls a callback function so the latest content of the file is read, mapped to a struct and then a corresponding Command/ScheduledTask is updated.

With the distributed server architecture, however, file-based configuration management is sometimes less preferred. Multiple servers may subscribe to a centralized configuration management system and reflect changes in a more real-time manner. HashiCorp's Consul, LINE's Central Dogma or some similar system can be used for such purpose.

The current wathcers.Watcher should be redefined so the current implementation and some other implementations that subscribe to such systems introduced above can co-exist.