Move processRaft goroutine from range to store.

This saves the cost of one goroutine per range and fits better with the
multiraft interface (which delivers all raft commits over a single
channel)

Also contains some test fixes as a followup to 96929484.
Fix a race and a golint error.
Merge pull request #171 from bdarnell/raft-to-store

Move processRaft goroutine from range to store.