search for: metadata_item_key

We unconditionally search for the EXTENT_ITEM_KEY for metadata during balance, and then check the key that we found to see if it is actually a METADATA_ITEM_KEY, but this doesn''t work right because METADATA is a higher key value, so if what we are looking for happens to be the first item in the leaf the search will dump us out at the previous leaf, and we won''t find our item. So instead do what we do everywhere else, search for the skinny...