Support batching in Redis

[alexsnaps]

Based off #269 we should look into supporting batch increments in server-side Lua.
Just like we do support batch read in VALUES_AND_TTLS, if we can do batch increments, the Batcher could make piggyback on the writes, retrieving the values (and their TTLs?) so to update the local cache with fresh values. All that asynchronously, so to avoid the overhead in band with the user's initial request.