Test sending an update after reconnecting

* Add `make test-gomc` for testing with the libmemcached adapter.
* Ensure `GomemcStore#fetchKey()` returns an empty record if the channel does not exist. This fixes a nil pointer exception thrown by `#Update()`.
* Add hooks for overriding duplicate client checks via `setReplaceEnabled()` and `replaceEnabled()`.
Why was this changed around?