Make the cs_sendProcessing a LOCK instead of a TRY_LOCK

Technically cs_sendProcessing is entirely useless now because it
is only ever taken on the one MessageHandler thread, but because
there may be multiple of those in the future, it is left in place

src/net.cpp

@@ -1871,8 +1871,7 @@ void CConnman::ThreadMessageHandler()
 
             // Send messages
             {
-                TRY_LOCK(pnode->cs_sendProcessing, lockSend);
-                if (lockSend)
+                LOCK(pnode->cs_sendProcessing);
                 GetNodeSignals().SendMessages(pnode, *this, flagInterruptMsgProc);
             }
             if (flagInterruptMsgProc)