channelstream: fix dogpile core import.

dogpile.core is depracated and will be removed

rhodecode/lib/channelstream.py

@@ -25,7 +25,7 b' import logging'
 import requests
 import datetime
 
-from dogpile.core import ReadWriteMutex
+from dogpile.util.readwrite_lock import ReadWriteMutex
 from pyramid.threadlocal import get_current_registry
 
 import rhodecode.lib.helpers as h