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.

@spencerkimball @cockroachdb/developers 
Because the first thing Start does is call Stop, so Stop has to leave the store in a startable state. Seems fishy, but that's how it works for now. We should clean this up in a separate commit.