Modify parallel pg_restore to track pending and ready items by means of

Modify parallel pg_restore to track pending and ready items by means of
two new lists, rather than repeatedly rescanning the main TOC list.
This avoids a potential O(N^2) slowdown, although you'd need a *lot*
of tables to make that really significant; and it might simplify future
improvements in the scheduling algorithm by making the set of ready
items more easily inspectable.  The original thought that it would
in itself result in a more efficient job dispatch order doesn't seem
to have been borne out in testing, but it seems worth doing anyway.