OPENDJ-2396 Replication: post-external-initialization generates an error in log

After online import or restore, replication startListenService() was being called twice, once from import handling and once from the callback registered by replication on backend changes.
While stopping the listener thread is idempotent, starting it was not, leading to contention by both threads.

Fix by forcing idempotency also when starting the listener thread and make sure idempotency also applie to the broker thread.