net: remove cs_vRecvMsg

vRecvMsg is now only touched by the socket handler thread.

The accounting vars (nRecvBytes/nLastRecv/mapRecvBytesPerMsgCmd) are also
only used by the socket handler thread, with the exception of queries from
rpc/gui. These accesses are not threadsafe, but they never were. This needs to
be addressed separately.

Also, update comment describing data flow

src/net.h

@@ -613,8 +613,6 @@ public:
     size_t nProcessQueueSize;
 
     std::deque<CInv> vRecvGetData;
-    std::list<CNetMessage> vRecvMsg;
-    CCriticalSection cs_vRecvMsg;
     uint64_t nRecvBytes;
     std::atomic<int> nRecvVersion;
 
@@ -726,6 +724,7 @@ private:
     const ServiceFlags nLocalServices;
     const int nMyStartingHeight;
     int nSendVersion;
+    std::list<CNetMessage> vRecvMsg;  // Used only by SocketHandler thread
 public:
 
     NodeId GetId() const {
@@ -746,7 +745,6 @@ public:
         return nRefCount;
     }
 
-    // requires LOCK(cs_vRecvMsg)
     bool ReceiveMsgBytes(const char *pch, unsigned int nBytes, bool& complete);
 
     void SetRecvVersion(int nVersionIn)