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()`.
After the [`IsClosed` check](https://github.com/mozilla-services/pushgo/blob/feature/test-renotify/src/github.com/mozilla-services/pushgo/simplepush/server.go#L275-278) to avoid removing the new client. Seems like there should be a better way...what do you think?