EXPERIMENTAL: Dedicated server idling system

- If no clients at server start or after 10 seconds of GS_LEVEL, and no Netxcmd waiting to be digested, halt all SV_MakeTic.
- Currently #define'd out, but if we don't get to test it before 1.6 release, I fully encourage community build developers to enable this codepath and trial it on their servers.
- It's absolutely netsafe to only have enabled on the host's end, the only risk is that a dedicated server might not re-awaken when presented with certain stimuli.