Respawns should be timed individually

Steps to reproduce:

• Find a bunch of respawning actors, e.g. outdoor guards.
• Kill one of them, leave the cell, wait for 48 hours.
• Re-enter the cell, kill another one, leave the cell, wait for 24 hours.
• Come back and observe.

Expected behaviour: the first guard should be respawned, the second one should be dead still. Instead, both guards have respawned.

Findings with corpse clearing in the original save files: The clear is tied to "dayspassed" variable; changing day or gamehour has no effect. Each creature has a clear countdown attached to it (see updated ACSC/ACDT structs).

(RM-2369 from redmine: created on 2015-02-14 by scrawl ., , closed on 2016-02-28 by scrawl ., blocks #2363 (closed))