Heartbeat on acquired locks copy

Instead of heartbeating on a collection that may
be changing size while iterating ensure that we only
iterate over a copy (that won't change size) during
iteration.

The python set() docs say that it is inherently
unpredictable to iterate on a set while its being mutated
so we should try to avoid that kind of case if we can.

Change-Id: I6f103a5528b3f62f1c8d9e501e8cadff4221ee40

tooz/drivers/redis.py

@@ -434,7 +434,7 @@ class RedisDriver(coordination.CoordinationDriver):
                 expiry_ms = max(0, int(self.membership_timeout * 1000.0))
                 self._client.psetex(beat_id, time_ms=expiry_ms,
                                     value=b"Not dead!")
-        for lock in self._acquired_locks:
+        for lock in self._acquired_locks.copy():
             try:
                 lock.heartbeat()
             except coordination.ToozError: