Similar to JGRP-1674.

flush_unblock_promise in FLUSH is not reset before use in waitForUnblock.

Any UNBLOCK (such as another node running FLUSH) will set it, not just the expected UNBLOCK.
The next call to waitForUnblock will not block.
(and if the UNBLOCK it was waiting for comes later, it will trigger the issue again on the next waitForUnblock).