net: add a flag to indicate when a node's process queue is full

Messages are dumped very quickly from the socket handler to the processor, so
it's the depth of the processing queue that's interesting.

The socket handler checks the process queue's size during the brief message
hand-off and pauses if necessary, and the processor possibly unpauses each time
a message is popped off of its queue.

src/net_processing.cpp

@@ -2475,6 +2475,8 @@ bool ProcessMessages(CNode* pfrom, CConnman& connman, std::atomic<bool>& interru
                 return false;
             // Just take one message
             msgs.splice(msgs.begin(), pfrom->vProcessMsg, pfrom->vProcessMsg.begin());
+            pfrom->nProcessQueueSize -= msgs.front().vRecv.size() + CMessageHeader::HEADER_SIZE;
+            pfrom->fPauseRecv = pfrom->nProcessQueueSize > connman.GetReceiveFloodSize();
             fMoreWork = !pfrom->vProcessMsg.empty();
         }
         CNetMessage& msg(msgs.front());