[#690] Fix finalizeWorkQueue never cancelling queued operations

[vharseko]

Problem

TraditionalWorkQueue.finalizeWorkQueue() is supposed to drain the pending operation queue at shutdown and send a "server shutting down" cancel response to every queued operation. Instead it calls:

ArrayList<Operation> pendingOperations = newArrayList<>();
opQueue.removeAll(pendingOperations);

Collection.removeAll(c) removes from the queue the elements contained in the argument — and the argument is a freshly created empty list, so nothing is removed and the list stays empty. removeAll never populates its argument, so the abort loop below is dead code: operations still queued at shutdown are silently dropped and their clients never receive a response (they wait until the connection is torn down).

The bug dates back to the original OpenDS implementation — the same pattern is present in the pre-OpenDJ-3 TraditionalWorkQueue — and went unnoticed because the queue is usually empty at shutdown.

Fix

Use opQueue.drainTo(pendingOperations), which moves all queued operations into the list so that each one gets aborted with the shutdown CancelRequest, as the surrounding code and comments intend.

Verification

TraditionalWorkQueueTestCase: 6/6 pass.

Fixes#690