Fix data race when applying replacements

Mutator context goroutines appending directly to the global Context's
replacements list causes a data race.  Send them over a channel
instead.

The renames and replacements are local to the mutator, so move them
out of Context and into the runMutator method.

Change-Id: I797edb1e27ee29f8946c58101b40fcfb50a32eb9