Fix missed diff in FoodMetaData. Fixes BUKKIT-4704

Prior to this commit the exhaustion of a player was not decreased by 3
like in vanilla.

This commit adds the missing exhaustion penalty and restores vanilla behaviour.
In order to preserve the functionality of cancelling a healing event, the
CraftBukkit constructed heal(float, RegainReason) was altered to return a
boolean for success as opposed to being a void method, this was done in
order to allow for the exhaustion penalty to only be issued when the
healing event is not cancelled.