Parallelize BottomUpMutators

Allow BottomUpMutators to run in parallel by calling Parallel() on the
return value of RegisterBottomUpMutator.  To avoid locking, moves
updates of global state into a separate goroutine that receives updates
over channels from the mutator goroutines.

Change-Id: Ic59b612da9b406cf59ec44940f0c1dee0c051a51