Makefile: Add a `test-gomc` task.
Protect against multiple `RemoveClient()` calls.

Fixes a race condition caused by a client reconnecting with the same ID
and `replaceWorkers` enabled. In some cases, `Serv.Bye()` was called
for the old socket after the client reconnected, removing the new
socket from the client map.
memcached: `fetchRec()` should never return nil.

Fixes a nil pointer exception in the gomemcache adapter.