Tweak terminate() slightly

* Gave it a default argument, since we never actually care about it, but
it needs to have it to conform to the signal handler prototype
* Constricted the area locked by activeCrit to only the block that deals
with the connections vector, to lower the chance of a future badly
placed call to terminate() deadlocking the server instead

src/Monitor.cpp

@@ -123,7 +123,7 @@ bool Monitor::acceptConnection(SOCKET fd, uint16_t revents) {
 
     if (revents & ~POLLIN) {
         std::cout << "[FATAL] Error on monitor listener?" << std::endl;
-        terminate(0);
+        terminate();
     }
 
     int sock = accept(listener, (struct sockaddr*)&address, &len);