Allow client handshake to be reset by server to support BungeeCord.

Added 'FMLHandshakeMessage.HandshakeReset' in order to allow servers, such as BungeeCord, to reset a client's handshake in order to change servers without a disconnect. FMLHandshakeClientState will simply listen for this packet in the 'DONE' state and transition to 'HELLO' when it receives it.