Bug 22456: Add cancellation request table

Bug 22456: Add cancellation request table

This patch adds a new table, for storing holds cancellation requests,
and managing them afterwards.

The *hold_id* attribute is not made a FK on the reserves table because
of the old_reserves thing we still do (see bug 25260 for a discussion
about it).

Sponsored-by: Montgomery County Public Libraries
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>