Add "Reduce Polling Rate" option to NetPlay

Normally, SI is polled at a rate defined by the game, and we have to send the pad state to other clients on every poll or else we'll desync. This can result in fairly high bandwidth usage, especially with multiple controllers, mostly due to UDP/IP overhead.

This change introduces an option to reduce the SI poll rate to once per frame, which may introduce up to one frame of additional latency, but will reduce bandwidth usage substantially, which is useful for users on very slow internet connections.

Polling SI less frequently than the game asked for did not seem to cause any problems in my testing, so this should be perfectly safe to do.

Source/Core/Core/NetPlayProto.h

@@ -27,6 +27,7 @@ struct NetSettings
   bool m_DSPEnableJIT;
   bool m_WriteToMemcard;
   bool m_CopyWiiSave;
+  bool m_ReducePollingRate;
   bool m_OCEnable;
   float m_OCFactor;
   ExpansionInterface::TEXIDevices m_EXIDevice[2];