Set pd_prune_xid on insert

Set pd_prune_xid on insert

Now that on-access pruning can update the visibility map (VM) during
read-only queries, set the page’s pd_prune_xid hint during INSERT and on
the new page during UPDATE.

This allows heap_page_prune_and_freeze() to set the VM the first time a
page is read after being filled with tuples. This may avoid I/O
amplification by setting the page all-visible when it is still in shared
buffers and allowing later vacuums to skip scanning the page. It also
enables index-only scans of newly inserted data much sooner.

As a side benefit, this addresses a long-standing note in heap_insert()
and heap_multi_insert(): aborted inserts can now be pruned on-access
rather than lingering until the next VACUUM.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com