pgsql: Add fake LSN support to hash index AM.

Add fake LSN support to hash index AM.

Use fake LSNs in all hash AM critical sections that write a WAL record.
This gives us a reliable way (a way that works during scans of both
logged and unlogged relations) to detect when an index page was
concurrently modified during the window between when the page is
initially read (by _hash_readpage) and when the page has any known-dead
items LP_DEAD-marked (by _hash_kill_items).

Preparation for an upcoming patch that makes the hash index AM use the
amgetbatch interface, enabling I/O prefetching during hash index scans.

The amgetbatch design imposes certain rules on index AMs with respect to
how they hold on to index page buffer pins (at least in the case of pins
held as an interlock against unsafe concurrent TID recycling by VACUUM).
These rules have consequences for routines that set LP_DEAD bits on
index tuples from an amgetbatch index AM: such routines have an inherent
need to reason about concurrent TID recycling by VACUUM, but can no
longer rely on their amgettuple routine holding on to a buffer pin
(during the aforementioned window) as an interlock against such
recycling.  Instead, they have to follow a new, standardized approach.

The new approach taken by amgetbatch index AMs when setting LP_DEAD bits
is heavily based on the current nbtree dropPin design, which was added
by commit 2ed5b87f.  It also works by checking if the page's LSN
advanced during the window where unsafe concurrent TID recycling might
have taken place.

This commit is similar to commit 8a879119, which taught nbtree to use
fake LSNs to improve its dropPin behavior.  However, unlike that commit,
this is not an independently useful enhancement, since hash doesn't
implement anything like nbtree's dropPin behavior (not yet).

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-WzkehuhxyuA8quc7rRN3EtNXpiKsjPfO8mhb+0Dr2K0Dtg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e5836f7b7d9a2949414ae022c74c18070202d429

Modified Files
--------------
src/backend/access/hash/hash.c       |  21 +++++---
src/backend/access/hash/hashinsert.c |  20 ++++---
src/backend/access/hash/hashovfl.c   | 100 ++++++++++++++++++-----------------
src/backend/access/hash/hashpage.c   |  22 ++++----
4 files changed, 91 insertions(+), 72 deletions(-)