Don't do periodic heal ticks if the target is already at max health.

If a target is at maximum health already periodic healing spells should
not tick.

From the healer's point of view this means far less overhealing, improving
their healing performance information.

Fixes cmangos/issues#385.
Merge branch 'periodic_heal_max' of https://github.com/Patman64/mangos-classic into develop