XLog size reductions: Reduced XLog record header size for PG17

Hi,

As I've mentioned earlier on-list [0][1] and off-list, I've been
working on reducing the volume of WAL that we write. This is one
intermediate step towards that effort.

Attached is a patchset that reduces the storage overhead of each WAL
record, with theoretical total savings in pgbench transactions
somewhere between 4-5%, depending on platform and the locality of
updates. These savings are achieved by reducing the amount of data
stored, and by not emitting bytes that don't carry data relevant to
each record's redo- and decode routines.

Patches contained:
0001 is copied essentially verbatim from [1] and reduces overhead in
the registered block's length field where possible. It is included to
improve code commonality between varcoded integer fields. See [1] for
more details.

0002 updates accesses to the rmgr-managed bits of xl_info with its own
macro returning only rmgr-managed bits, and updates XLogRecGetInfo()
to return only the xlog-managed bits.

0003 renames the rm_identify argument from 'info' to 'rmgrinfo'; and
stops providing the xlog-managed bits to the function - rmgrs have no
need to know the xlog internal info bits.

0004 continues on 0003 and moves the rmgr info bits into their own
xl_rmgrinfo of type uint8, stored in the alignment hole in the
XLogRecord struct.

0005 updates the code to only include a valid XID in the record when
the rmgr actually needs to use that XID.

0006 implements a new, variable length, WAL record header format,
previously discussed at [0] and [2]. This new WAL record header is a
minimum of 14 bytes large, but generally will be 15 to 21 bytes in
size, depending on the data contained, the type of record, and whether
the record needs an XID.

Notes:
- The patchset includes [1] for its variable-length encoding of
uint32, and this is included in the savings calculation.
- Not all records include the backend's XID anymore. XLog API users
must explicitly request the inclusion of XID in the record with the
XLOG_INCLUDE_XID record flag.
- XLog records are now always aligned to 8 bytes. This was needed to
reduce the complexity of var-coding the record length on 32-bit
systems. Savings on 32-bit systems still exist, but can be expected to
be less impactful.
- XLog length is now varlength encoded. No more records with <255
bytes of data storing 3 0-bytes - the length is now stored in 0, 1, 2
or 4 bytes.
- RMGRs now get their own uint8 info/flags field. No more sharing bits
with WAL infrastructure in xl_info. The byte is only stored if it is
non-0, and otherwise omitted (indicated by flag bits in xl_info).

Todo:
- Check for any needed documentation / code comments updates
- benchmark this

Future work:
- we could omit XLR_BLOCK_ID_DATA_[LONG,SHORT] if it is the only
"block ID" in the record (such as checkpoint records, commit/rollback
records, etc.). This would be indicated by a xl_info bit, and this
would save 2-5 bytes per applicable record.
- This patch inherits [1]'s property in which we can release the
BKPBLOCK_HAS_DATA flag bit (its value is already implied by
XLR_BLOCKID_SZCLASS), allowing us to use it for something else, like
indicating varcoded RelFileLocator/BlockId.

Kind regards,

Matthias van de Meent
Neon, Inc.

[0]
https://www.postgresql.org/message-id/flat/CAEze2Whf%3DfwAj7rosf6aDM9t%2B7MU1w-bJn28HFWYGkz%2Bics-hg%40mail.gmail.com
[1] https://www.postgresql.org/message-id/flat/CAEze2WjuJqVeB6EUZ1z75_ittk54H6Lk7WtwRskEeGtZubr4bQ%40mail.gmail.com
[2] https://www.postgresql.org/message-id/flat/CA+Tgmoaa9Yc9O-FP4vS_xTKf8Wgy8TzHpjnjN56_ShKE=jrP-Q@mail.gmail.com