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.

A small amount of related cleanup has also been done.
Avoid NPEs when a REGISTER packet is sent earlier than expected, don't store sent REGISTER messages
Strip out REGISTER messages that have already been sent when sending the cached versions
Merge pull request #3 from dualspiral/fml_register

Patch FML Registering.