Reliable multicast RPCs can be dropped

As detailed in the linked UDN thread, if a server doesn't receive a network packet from a client in 1.5 seconds, it sets the client connection's ViewTarget to null. This will cause dropped reliable multicast RPCs due to a ViewTarget null check on in UIpNetDriver::ProcessRemoteFunction. Probably can't remove that check due to the issue in this thread: https://answers.unrealengine.com/questions/97569/client-cant-join-a-server-if-server-is-running-rel.html

Should at least make the 1.5 seconds configurable, and maybe consider whether we need it at all.