Add 'RedisPubSubFeedEngine' feed engine

This patch adds a class which implements the RCFeedEngine interface by
publishing recent change notifications to Redis. The class handles the
'redis://' URI scheme. Recent changes are PUBLISHed to the channel 'rc'; a
different channel name may be specified as a path component.

Change-Id: I846036c091c45059a8947245a1efe92c9800dcf4
Merge "Add 'RedisPubSubFeedEngine' feed engine"