bug 1552492, make build-retention command not shoot us on restarts.
bug 1552492, make build-retention command not shoot us on restarts.

BuildRequest objects are really only important on build start, so I'm keeping
track of the last build request which is bound to one of the builds I delete
in retention. Then I delete all orphaned BuildRequest objects up to that one.

I also hook up clean_builds, but I tolerate that failing.

mozilla/elmo,mozilla/elmo,mozilla/elmo,mozilla/elmo