[gocbcore] Enhanced durability can be incorrectly flagged as unsupported

When performing KV ops using enhanced durability without using WaitUntilReady it's possible to run into an unsupported features error. This is because when we setup the initial kv mux state we see that the fake config doesn't have support so mark it as unsupported until we see a config that says otherwise. This should remain unknown until we see an actual config.