Convert pg_restore's ready_list to a priority queue.

Presently, parallel restores spend a lot of time sorting this list
so that we pick the largest items first.  With many tables, this
sorting can become a significant bottleneck.  There are a couple of
reports from the field about this, and it is easy to reproduce.

This commit improves the performance of parallel pg_restore with
many tables by converting its ready_list to a priority queue, i.e.,
a binary heap.  We will first try to run the highest priority item,
but if it cannot be chosen due to the lock heuristic, we'll do a
sequential scan through the heap nodes until we find one that is
runnable.  This means that we might end up picking an item with a
much lower priority.  However, we expect that we will typically be
able to pick one of the first few items, which should usually have
a relatively high priority.

Suggested-by: Tom Lane
Tested-by: Pierre Ducroquet
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/3612876.1689443232%40sss.pgh.pa.us

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