Comment 8 for bug 1787274

Patches:

http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/berick/lp1787274-checkin-dupes

1. Adds a check to the browser checkin UI to ensure a given copy may only have one in-flight checkin API call at a time.

2. Adds unique constraint triggers to action.transit_copy / hold_transit_copy / reservation_transit_copy. Because of the inheritance relationship between these tables, a simple unique index would not suffice.

3. Adds an index on target_copy where dest_recv_time and cancel_time are null, i.e. "open transit for copy". This may not be necessary, but since the check constraint makes this exact query, as well as parts of the application, it seemed like a win.

4. PGTAP tests.