LP#1949910: serialize deleting items from item bucket

LP#1949910: serialize deleting items from item bucket

This patch serializes the fetching and fleshing of items to delete
when the delete-from-item-bucket action is invoked, thereby
avoiding possible drone starvation from open-ils.search. It
also throws up the progress dialog for good measure while
the deletion occurs.

To test
-------
[1] Set up an item bucket with 25-50 items.
[2] From the item bucket intervace, invoke the action to delete
    all of the items.
[3] Note that the open-ils.search.asset.copy.fleshed2.retrieve calls,
    one for each item, are all made simultaneously. Assuming stock
    max_children settings, this will cause open-ils.search to log
    that some requests are hitting the backlog.
[4] Apply the patch and refresh the item bucket page, then repeat
    step 2 (it doesn't matter for this purpose that the items have
    already been deleted). This time, the open-ils.search API calls
    are made serially and a progress bar is displayed while the
    deletion takes place. Verify that no requests hit the backlog.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>