Revisit ReconnectNodeRemover synchronization

[artemananiev]

This issue was found during investigation for #9764.

Current node remover threading model is as follows:

• LearnerThread receives all the nodes from the teacher
• On the same thread, nodes are deserialized and put to a) node remover b) reconnect hasher
• The remover doesn't handle the nodes right away, but puts them to a queue
• The queue is processed on a different thread
• Once LearnerThread is done with its main loop, the node remover is closed. Some elements may still be unprocessed in the queue
• In the fix for ReconnectNodeRemover.getRecordsToDelete() loops forever #9764 a call to process remaining elements in the queue was added

After #9764, it seems to work, but synchronization is still fuzzy. For example, if the hasher calls ReconnectNodeRemover.getRecordsToDelete(), and the remover is closed in a parallel threads, the queue thread is stopped, but there are calls to workQueue.pause() and workQueue.resume() in getRecordsToDelete(). What do these calls do if the queue thread is stopped? Probably, no-ops + error log messages, probably not.

Is a separate queue thread needed in node remover at all? Can node requests be handled right on the learner thread? This ticket is to find answers to questions like this.

[artemananiev]

ReconnectNodeRemover has been re-worked completely as a part of #11231, it now doesn't use any queues or threads. This issue is no longer relevant.