auto-reload configuration when config files change

[dhiaayachi]

This add the ability to auto-reload config based on config file change. use FileWatcher merged in #12301.

Missing tests, but ready for a first round of review.

[rboyer]

trivial:
These could be merged into a single conditional and an entire level of indentation removed.

[rboyer]

(purely optional; codegolf)
You could move just the interaction of the lock and the map into some helpers to keep them tighter, like:

func (w *fileWatcher) replaceFile(oldFile, newFile string, modTime time.Time) {
	w.configFilesLock.Lock()
	defer w.configFilesLock.Unlock()
	delete(w.configFiles, oldFile)
	w.configFiles[newFile] = &watchedFile{modTime: modTime}
}

func (w *fileWatcher) addFile(newFile string, modTime time.Time) {
	w.configFilesLock.Lock()
	defer w.configFilesLock.Unlock()
	w.configFiles[filename] = &watchedFile{modTime: modTime}
}

func (w *fileWatcher) removeFile(oldFile string) {
	w.configFilesLock.Lock()
	defer w.configFilesLock.Unlock()
	delete(w.configFiles, oldFile)
}

[rboyer]

LGTM. Only two non-blocking comments.

[hc-github-team-consul-core]

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