Mobs no longer immediately freeze when an area is empty

[Rias]

Per the title/changelog: Mobs will no longer immediately freeze when an area is empty of PCs, instead continuing to do their thing for a short while after the area has become considered empty.

This should have a number of beneficial results, including:

- Agitated mobs will have time to calm down when an area is empty.
- Mobs that had clumped together in rooms to attack PCs will have time to break up their clump as individuals have chances to move into different rooms. (Note that there are rare cases of mobs deliberately grouping up.)
- Mobs that can close and lock opened portals will have more time to do that and prepare for the next home invasion to come their way.
- Mobs that are programmed to "seek healing" or "seek rest" (despawn) under the right conditions will now have more of a chance to do that instead of patiently waiting for PCs to re-enter the area and finish them off.
- In general, it'll be less goofy to come back to an area and not see all the mobs in exactly the same positions they were in before.

If anyone thinks of or observes any potentially unintended side-effects resulting from this update, please report it here.