NATS Config hot-reload using file watcher

[AloysA]

We currently use and love NATS, but we deploy our software on Windows in sensitive customer environments. NATS and our own applications run as Windows Services with limited privileges. This eliminates our chances of doing a config reload, because that requires administrative privileges using nats-server.exe --signal reload.

I propose a new feature in which the configuration can be hot-reloaded via file watching. This is not uncommon and could be easily opt-in by specifying in the configuration file itself whether hot-reloading the config file based on a file watcher should be enabled or not.

By using this mechanism we can modify the NATS configuration if our system dynamically changes without needing the administrative privileges. I believe this might suit more Windows users of NATS.

(Unfortunately I have zero experience with Go, so creating a pull request myself is too far reached at this time).

It seems that file watcher can be implemented cross platform relatively easily using: https://github.com/fsnotify/fsnotify