Allow SIP checkout to pre-empt unfilled holds.

Allow SIP checkout to pre-empt unfilled holds.

This affects transactions on items that may be covered by
a title or item-level hold, but have not yet been retrieved
from the stacks (i.e. "confirmed") by the librarian. This does
not affect waiting holds (found="W"), so they will still block
transaction unless they belong to the operating patron.

Also I cleanup any hold the patron has on a biblio/item when
they are allowed to checkout the item.
Here we are filling the hold because the checkout is allowed, regardless
of of the queue position.  This is different from AddIssue's CancelReserve, that
only fills the hold if it is next in line, but in the future AddIssue should
adopt a similar logic.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>