Update to latest batsd

There have been very few changes to upstream since we forked. The reason we're doing this is to pick up the delete key feature so we can cherry pick deleting specific keys instead of having to nuke the whole DB. 

The only other change of consequence is that they are using a separate thread pool for timers. Its the same threadpool class they were using before, they just created a new instance of it just for timers.  Seems very low risk.

Our changes are rebased on top. Take a look and let me know what you think.

@obrie @deanxorr @will-ob @mattcarbone