Fix potential deadlock

Conflict:
* cs_main in ProcessMessages() (before calling ProcessMessages)
* cs_vSend in CNode::BeginMessage
versus:
* cs_vSend in ThreadMessageHandler2 (before calling SendMessages)
* cs_main in SendMessages

Even though cs_vSend is a try_lock, if it succeeds simultaneously with
the locking of cs_main in ProcessMessages(), it could cause a deadlock.