Add :Clean() method to pool classes.

Add :Clean() method to pool classes.

This method can be used to release resources held in tables allocated from
the pool object as part of releasing the item back to the pool via
pool:Release(item).  The pool:Clean(item) method is called shortly before
the table is wiped and returned to the table pool.

git-svn-id: svn://svn.curseforge.net/wow/ovale/mainline/trunk@1184 d5049fe3-3747-40f7-a4b5-f36d6801af5f