Improve efficiency of the redis driver when posting messages

Rather than using a watch transaction to rank messages, this patch
modifies the Redis driver's message controller so that it will
use a Lua script. This will avoid retries and MessageConflict errors
under high concurrency.

Change-Id: Idb020ab5b949da5f0bbbfdf3919c217e18bfdc9e
Merge "Improve efficiency of the redis driver when posting messages"