refactor(fsnotifyext): remove unnecessary mutex usage from Deduper

[wazazaby]

Hello,

This is a small refactor of the Deduper component, here's a list of the introduced changes :

• Move the Deduper timers map to goroutine, to avoid mutex usage as it's read from / written to from a single goroutine.
• Deduper.GetChan now returns a receive only channel, as only the internal fsnotify.Watcher instance should be allowed to send events to it.
• Use fsnotify.Event.Has to check against Chmod operations (as per recommended by the fsnotify.Event.Op godoc).
• In Executor.watchTasks - remove check against Chmod operations, it's already handled in the Deduper.GetChan goroutine.
• Add a little bit of godoc on the Deduper.GetChan method to explain that.
• Run a go mod tidy.

Following the contributing guidelines, I will squash the commits upon merging (if it ever happens ^^).

Thank you :)