Reduce cleanup lock contention on standby replay

Hi Hackers,

On standbys, WAL replay of VACUUM-generated XLOG_HEAP2_PRUNE records
requires a cleanup lock (exclusive lock + zero pin count) on the target
buffer. If any backend holds a pin on that buffer, even from a trivial
SELECT, replay stalls until max_standby_streaming_delay expires, at which
point the query
is canceled. This can also cause increased replication lag.

Andres and me chatted about this problem offline and he suggested below
approaches to improve the current situation:

1. Shorten the time the queries hold the pins
2. Eliminate unnecessary cleanup lock requirements in replay
3. Improve diagnosis logging

Additionally, we discussed an approach of redo process performing COW on
cleanup lock conflict instead of waiting until the
max_standby_streaming_delay under a GUC).
I am not including it as part of this thread and can start a different one
if there is enough interest in that approach.

Attached 3 distinct patches for each of these.

Patch 1: Reduce buffer pin lifetime for unique index point lookups

The most common OLTP pattern is fetching a single row by primary key which
holds a heap buffer pin in the scan descriptor (xs_cbuf) until the next
fetch or scan end.
For a point lookup returning exactly one row, this means the pin lives for
the entire duration of whatever the executor does with that tuple
(evaluating expressions, passing
through plan nodes, pg_sleep() in a function, nested-loop outer side, etc.).

This patch adds two new ScanOption flags:
SO_MATERIALIZE_ON_FETCH (IndexScan) - After fetching the heap tuple,
materialize it into the slot (heap_copytuple equivalent) and release both
the slot's buffer pin and
the scan descriptor's xs_cbuf pin.

SO_RELEASE_PIN_ON_FETCH (IndexOnlyScan) - Since IOS only visits the heap
for visibility checks (output data comes from the index), just release
xs_cbuf.
The slot's own pin is released by ExecClearTuple immediately after
index_fetch_heap returns in IndexOnlyNext. No tuple copy is needed.

Both flags are set only when:
- The index is unique (rd_index->indisunique)
- Scan keys cover all key columns (NumScanKeys >= indnkeyatts)

This guarantees at most one heap page visit per scan, so there is no
same-page pin reuse benefit to preserve (unlike range scans where
consecutive tuples often share a buffer).
I didn't see any obvious difference in performance with this change
because the materialize cost (one heap_copytuple per point lookup) is
negligible compared to
the index traversal + buffer lock cycle.

Patch 2: Skip cleanup lock on replay for freeze-only prune records

Currently, all XLOG_HEAP2_PRUNE WAL records request a cleanup lock on
replay (XLHP_CLEANUP_LOCK flag is unconditionally set). However, when
a prune operation ONLY freezes tuples i.e. no redirections, no dead items,
no unused items then it doesn't modify line pointers. Concurrent readers
that follow line pointers are not affected by a freeze (which only touches
tuple header xid fields).

This patch passes `do_prune` instead of unconditional `true` as the
cleanup_lock parameter to log_heap_prune_and_freeze(). When do_prune is
false (freeze-only case),
the WAL record is emitted without XLHP_CLEANUP_LOCK. On replay, the
standby acquires only an exclusive content lock (not a cleanup lock),
eliminating recovery conflicts for
freeze-only operations. This is a targeted fix for a common scenario:
aggressive_freeze or vacuum_freeze_min_age=0 vacuums that freeze pages
without needing to prune dead tuples.

The heapdesc.c change adds "cleanupLock: T/F" to pg_waldump output for
prune records, making it easy to audit which records actually need cleanup
locks.

Patch 3: Add relation info to recovery conflict buffer pin logs

When a recovery conflict on a buffer pin is logged (either resolved or
triggering query cancellation), the current message only says "recovery
conflict on buffer pin" with timing information.
It doesn't tell which relation/page is causing the conflict.

This patch adds the buffer tag (spcOid/dbOid/relNumber/blockNum) to the LOG
message, so DBAs can identify:
- Which table is being vacuumed heavily
- Which specific block is hot
- Whether the conflict is on a heap page or index page

Example output:
LOG: recovery conflict waiting on buffer pin: rel 1663/16384/16385 blk 42

Please review and let me know if there are other cases to optimize and
additional cases to handle.

Thanks,
Satya