_bt_killitems marks btree items dead when a scan leaves the page where
they live, but it does so with only share lock (to improve concurrency).
This was historicall okay, since killing a dead item has no
consequences.  However, with the advent of data checksums and
wal_log_hints, this action incurs a WAL full-page-image record of the
page.  Multiple concurrent processes would write the same page several
times, leading to WAL bloat.  The probability of this happening can be
reduced by only killing items if they're not already dead, so change the
code to do that.

The problem could eliminated completely by having _bt_killitems upgrade
to exclusive lock upon seeing a killable item, but that would reduce
concurrency so it's considered a cure worse than the disease.

Backpatch all the way back to 9.5, since wal_log_hints was introduced in
9.4.

Author: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Discussion: https://postgr.es/m/CA+fd4k6PeRj2CkzapWNrERkja5G0-6D-YQiKfbukJV+qZGFZ_Q@mail.gmail.com