Lock TaskUpdateQueue.flush()

This way it can be called from multiple coroutines without getting race
conditions.

flamenco_worker/upstream_update_queue.py

@@ -29,6 +29,7 @@ class TaskUpdateQueue:
 
     _stuff_queued = attr.ib(default=attr.Factory(asyncio.Event), init=False)
     _db = attr.ib(default=None, init=False)
+    _queue_lock = attr.ib(default=attr.Factory(asyncio.Lock), init=False)
     _log = attrs_extra.log('%s.TaskUpdateQueue' % __name__)
 
     def _connect_db(self):
@@ -127,6 +128,7 @@ class TaskUpdateQueue:
         Returns True iff the queue was empty, even before flushing.
         """
 
+        with (await self._queue_lock):
             queue_is_empty = True
             for rowid, url, payload in self._queue():
                 queue_is_empty = False