config: setter/getter for Notifier and Logger into its own struct. (#3721)

This is an attempt cleanup code and keep the top level config
functions simpler and easy to understand where as move the
notifier related code and logger setter/getter methods as part
of their own struct.

Locks are now held properly not globally by configMutex, but
instead as private variables.

Final fix for #3700

cmd/logger-file-hook.go

@@ -35,7 +35,7 @@ type localFile struct {
 }
 
 func enableFileLogger() {
-	flogger := serverConfig.GetFileLogger()
+	flogger := serverConfig.Logger.GetFile()
 	if !flogger.Enable || flogger.Filename == "" {
 		return
 	}