Made the SecureEnvelope message part of the encrypted payload wherever possible. This applies to both the GC-GS and FC-FS interactions. It will help later on when protecting against active attackers. Added sequence numbers to all messages after the initial session initialization. Put the nonce return in the encrypted payload. Added a FileClientThread, which is the FC counterpart to the GroupClientThread. This deals with all incoming SecureEnvelopes and makes verifying sequence numbers more streamlined. Later this will make dealing with SecureEnvelope integrity verification easier as well.