P2P: parse network datastream into header/data components in socket thread

Replaces CNode::vRecv buffer with a vector of CNetMessage's.  This simplifies
ProcessMessages() and eliminates several redundant data copies.

Overview:

* socket thread now parses incoming message datastream into
  header/data components, as encapsulated by CNetMessage
* socket thread adds each CNetMessage to a vector inside CNode
* message thread (ProcessMessages) iterates through CNode's CNetMessage vector

Message parsing is made more strict:

* Socket is disconnected, if message larger than MAX_SIZE
  or if CMessageHeader deserialization fails (latter is impossible?).
  Previously, code would simply eat garbage data all day long.
* Socket is disconnected, if we fail to find pchMessageStart.
  We do not search through garbage, to find pchMessageStart.  Each
  message must begin precisely after the last message ends.

ProcessMessages() always processes a complete message, and is more efficient:

* buffer is always precisely sized, using CDataStream::resize(),
  rather than progressively sized in 64k chunks.  More efficient
  for large messages like "block".
* whole-buffer memory copy eliminated (vRecv -> vMsg)
* other buffer-shifting memory copies eliminated (vRecv.insert, vRecv.erase)

Conflicts:
	src/main.cpp
	src/net.h
Some fixes to CNetMessage processing

* Change CNode::vRecvMsg to be a deque instead of a vector (less copying)
* Make sure to acquire cs_vRecvMsg in CNode::CloseSocketDisconnect (as it
  may be called without that lock).
P2P, cosmetic: break out buffer send(2) code into separate function
P2P: improve RX/TX flow control

1) "optimistic write": Push each message to kernel socket buffer immediately.

2) If there is write data at select time, that implies send() blocked
   during optimistic write.  Drain write queue, before receiving
   any more messages.

This avoids needlessly queueing received data, if the remote peer
is not themselves receiving data.

Result: write buffer (and thus memory usage) is kept small, DoS
potential is slightly lower, and TCP flow control signalling is
properly utilized.

The kernel will queue data into the socket buffer, then signal the
remote peer to stop sending data, until we resume reading again.
Use per-message send buffer, rather than per connection

Conflicts:
	src/main.cpp
	src/net.h