Various NPC attack logic fixes (this is *really* messy).
Fixes death animation being sent twice on some occasions (not noticeable by the client anyway).
Fixes multiple OnDeath() handling.
Fixes NPCs losing double the damage (removed the manual HP update), making the HP bar reading inconsistent with the actual data; resolves #368.
Fixes NPC attacks on users not triggering the OnDeath() handler in rare circumstances; refs #357.
Fixes users being seen as alive after respawning when blinking's disabled.