Allow delay between watch errors

When a watcher error occurs (due to some key being
watched being mutated) the current behavior is to
immediately try again. To avoid the thundering herd
problem a delay is nice to provide to avoid these
situations by introducing a sleep period between these
types of failures.

redis/client.py

@@ -3,6 +3,7 @@
 import datetime
 import sys
 import warnings
+import time
 import threading
 import time as mod_time
 from redis._compat import (b, basestring, bytes, imap, iteritems, iterkeys,
@@ -477,6 +478,7 @@ def transaction(self, func, *watches, **kwargs):
         """
         shard_hint = kwargs.pop('shard_hint', None)
         value_from_callable = kwargs.pop('value_from_callable', False)
+        watch_delay = kwargs.pop('watch_delay', None)
         with self.pipeline(True, shard_hint) as pipe:
             while 1:
                 try:
@@ -486,6 +488,8 @@ def transaction(self, func, *watches, **kwargs):
                     exec_value = pipe.execute()
                     return func_value if value_from_callable else exec_value
                 except WatchError:
+                    if watch_delay is not None and watch_delay > 0:
+                        time.sleep(watch_delay)
                     continue
 
     def lock(self, name, timeout=None, sleep=0.1, blocking_timeout=None,