Added area refresh rules to JSON properties

[Synival]

Every zone's "area.json" should have custom settings to determine how it should refresh.

The following rules should be available:

• "min_reset_age": int
  The minimum number of area ticks necessary before an update is considered.
  Default: AREA_RESET_MINIMUM_AGE (3)

• "always_reset_age": int
  The maximum number of area ticks that can pass before an update occurs.
  Default: AREA_RESET_ALWAYS_AGE (31)

• "active_reset_age": int
  The maximum number of area ticks that can pass before an update occurs If players have been in (or are currently in) this zone before a reset occurs.
  Default: AREA_RESET_AFTER_PLAYERS_AGE (15)

To implement this, we'll need some new fields for struct area_data in structs.h:

int min_reset_age;
int always_reset_age;
int active_reset_age;

When creating new areas, area_init() in recycle.c should set the defaults to the #defines listed in the JSON description.

The area_should_update() function in areas.c should be modified to use the new fields instead of the hard-coded #defines.