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/notify-redis.go

@@ -83,7 +83,7 @@ func dialRedis(rNotify redisNotify) (*redis.Pool, error) {
 }
 
 func newRedisNotify(accountID string) (*logrus.Logger, error) {
-	rNotify := serverConfig.GetRedisNotifyByID(accountID)
+	rNotify := serverConfig.Notify.GetRedisByID(accountID)
 
 	// Dial redis.
 	rPool, err := dialRedis(rNotify)