Ensure stream:features have been received before consider them "processed"

I've found the issue working against Tigase server.

Tigase does not send the feature list on the first response after the connection, so the register plugin correctly calls "_no_auth_received" to send a "poll" request; however as the processed_features flag was being set, the register plugin never got to process the actual response with the features, and hence never registers the user.

With this change will ensure that the list was actually received before setting the flag.