commandserver: backport handling of forking server from chgserver This is common between chg and vanilla forking server, so move it to commandserver and unify handle(). It would be debatable whether we really need gc.collect() or not, but that is beyond the scope of this series. Maybe we can remove gc.collect() once all resource deallocations are switched to context manager.

commandserver: promote .cleanup() hook from chgserver This allows us to unify _requesthandler.handle().