Added new caching feature that remembers the amount of health your mount
had, and spawns the next mount for you with that amount of health.  That
means if you switch worlds you can respawn your mount with the same
amount of health.  This is also persistent between server restarts and
disconnects.  Your mount can be cached via /unmount, or it is cached
automatically if you are using the new config option to despawn the
mount when you despawn.

Also added forementioned config option.