Hold current_shelf_lib API for setting/clearing.

Hold current_shelf_lib API for setting/clearing.

Sets and clears the hold.current_shelf_lib value as appropriate during
hold capture, transit checkin, hold resetting, etc.

Also, we now create hold transits (instead of regular transits) when a
captured copy is (unexpectedly) found at the wrong branch and needs to
transit to the pickup lib.  This can happen if the captured copy simply
transits to the wrong place (existing behavior) or the pickup lib for
the hold was changed while the copy was on the shelf (new behavior).

Signed-off-by: Bill Erickson <berick@esilibrary.com>