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-amqp.go

@@ -59,7 +59,7 @@ func dialAMQP(amqpL amqpNotify) (amqpConn, error) {
 }
 
 func newAMQPNotify(accountID string) (*logrus.Logger, error) {
-	amqpL := serverConfig.GetAMQPNotifyByID(accountID)
+	amqpL := serverConfig.Notify.GetAMQPByID(accountID)
 
 	// Connect to amqp server.
 	amqpC, err := dialAMQP(amqpL)