Implement server draining

[pg9182]

• Send isDraining to masterserver.
• Expose drain to script.
• Exit gracefully on match end if draining.
• Exit gracefully after last player leaves if draining.
• Exit gracefully immediately if no players when draining.
• Exit gracefully after timeout if draining and timeout is enabled.
• Figure out a good place to put the drain status (concommand? global? etc?).
• Provide a way to trigger drain externally.

R2Northstar/Northstar#544
requires R2Northstar/Atlas#40

[r3muxd]

there is absolutely no point in this; this is replicated by running the command sv_shutdown, which will quit the server when the server next enters hibernation (which will occur when the server is empty)
to prevent new players from joining, the sv_rejectClientConnects cvar can be used

[GeckoEidechse]

to prevent new players from joining, the sv_rejectClientConnects cvar can be used

Haven't gotten around to check yet whether that convar exists. Nevertheless, we still need to communicate to master server that gameserver is no longer accepting new players. Simply refusing connection on join gives a bad user experience.

The whole point of the PR is to give a mechanism to reduce server playercount to zero to allow for a restart without interrupting player experience. If done on a round end / map change, it should at least be announced to players that server will go down at the end of round.

Server draining (maybe even through an external trigger) is desired to allow for future updating mechanisms of servers that does not interrupt active sessions.