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()`.
While this will remove the prior worker from the app mapping and allow the new one on, it doesn't actually forcibly disconnect the worker allocated to the uaid. It should probably close the actual socket of the prior client to force its immediate termination (and the goroutine associated with it).