Log maximum health for horses

Horses have different maximum healths (15 - 30 HP according to the wiki). Prism does not at the moment log horses' maximum health. If a horse death is rolled back, the horse in question has its maximum health set to 52 HP (or 26 hearts).

This PR stores the maximum health in the database whenever a horse is killed, and uses that value when rolling back a horse. This way, the horse's maximum health is what it was before it was killed instead of 52.