[Forge] Allow passing of REGISTER messages from Client to Server at the right moment during the FML Handshake.

Some mods rely on the channels being registered before Bungee normally gets around to it. This commit allows the required packets to go through, which are the ones in response to the ServerHello message. Without this, the REGISTER packets would be sent later - too late for some mods that rely on it.

I have tested this through debugging, the REGISTER messages are now being sent/passed on at the right time. Thanks to @boq for alerting me to this issue in #1248. Though I don't know if this is the cause of #1249 - this may fix that too.

A small amount of related cleanup has also been done.