[Backport MB-39163 to 6.5.2] Avoid possible deadlock in removeListener in gometa leader

Whenever a listener is being removed from the leader, a notification is sent to leader's change channel. This is done while holding a lock. If the channel is full, the function will block here.

On the other side, if leaderServer is processing a notification on leader's change channel, It will call GetActiveEnsembleSize which also attempts to take same lock (which is held by removeListener), and hence leading to a deadlock.