LP#1795906: new global flag to toggle use of pickup lib in hold queue calculation

LP#1795906: new global flag to toggle use of pickup lib in hold queue calculation

This patch adds a new global flag, circ.holds.queue_position_relative_to_pickup_lib,
to control whether or not the pickup library is used to restrict the set of
holds constituting the queue that a given hold belongs to. When this flag is
enabled (as it is by default), the queue includes only holds with the same
pickup library. If the flag is not enabled, the previous behavior of including
all relevant holds in the entire consortium is used instead.

This flag is provided in cases where material is broadly shared among the
entire consortium and few of the options to have holds prefer to be
filled locally are in force. In that case, to display a queue position
relative to the pickup library may cause patrons to think that their
requests will be filled significantly sooner than they should expect.

To test
-------
[1] Create five holds on one title for five different patrons, with the
    first with pickup library BR1 and the last with pickup library BR2.
[2] View the hold in the My Account page for the fifth patron. The hold
    should display as #5.
[3] Apply the patch and repeat step 2. The hold should display as #1.
[4] Disable the new global flag. The hold should display as #5 again.

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