Reserve slots for all players on map change

[crescentrose]

This gives all players connected after a match ends a configurable grace period (60 seconds by default) to join the next map. This ensures they will not be kicked in case the server is temporarily over capacity due to a person with a reserved slot being connected, or by being "ninja'd" out of their slot.

Configure by changing the sm_reserve_grace convar. Set to 0 to disable this feature.

This overrides the default reservedslots.sp plugin.

Needs to be tested, don't merge yet.