maintner: remove Corpus.StartLogging, rename processMutation to addMutation

StartLogging seemed unnecessary. And processMutation was special in
that it added to the log. The other dozen or so methods named
processMutation or similar were just about process mutations already
in the log. Disambiguate.

Change-Id: I4b4bf46c3ae7996ad5f7e06de5456f4c06b6bb40
Reviewed-on: https://go-review.googlesource.com/38658
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
6 files changed