Don't check connection state in Locker#assert!

Rely on the call to zk.exists? to check that.

I've noticed that the threaded connection has a `:retry_duration` which would allow for temporary disconnections to be recovered from, but this check to `zk.connected?` prevents that from working.

This way, we can let the call to `zk.exists?` do the job of checking if we're connected, and it will place nice with reconnection.