pgsql: Rename wal_keep_segments to wal_keep_size.

Rename wal_keep_segments to wal_keep_size.

max_slot_wal_keep_size that was added in v13 and wal_keep_segments are
the GUC parameters to specify how much WAL files to retain for
the standby servers. While max_slot_wal_keep_size accepts the number of
bytes of WAL files, wal_keep_segments accepts the number of WAL files.
This difference of setting units between those similar parameters could
be confusing to users.

To alleviate this situation, this commit renames wal_keep_segments to
wal_keep_size, and make users specify the WAL size in it instead of
the number of WAL files.

There was also the idea to rename max_slot_wal_keep_size to
max_slot_wal_keep_segments, in the discussion. But we have been moving
away from measuring in segments, for example, checkpoint_segments was
replaced by max_wal_size. So we concluded to rename wal_keep_segments
to wal_keep_size.

Back-patch to v13 where max_slot_wal_keep_size was added.

Author: Fujii Masao
Reviewed-by: Álvaro Herrera, Kyotaro Horiguchi, David Steele
Discussion: https://postgr.es/m/574b4ea3-e0f9-b175-ead2-ebea7faea855@oss.nttdata.com

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f5dff45962ec0a0daad443e45811d6c426be1237

Modified Files
--------------
doc/src/sgml/catalogs.sgml                    |  2 +-
doc/src/sgml/config.sgml                      | 21 ++++++++++---------
doc/src/sgml/high-availability.sgml           |  4 ++--
doc/src/sgml/ref/pg_basebackup.sgml           |  2 +-
doc/src/sgml/wal.sgml                         |  3 ++-
src/backend/access/transam/xlog.c             | 30 ++++++++++++++++-----------
src/backend/replication/slotfuncs.c           | 13 ++++++------
src/backend/utils/misc/guc.c                  | 11 +++++-----
src/backend/utils/misc/postgresql.conf.sample |  2 +-
src/bin/pg_rewind/t/RewindTest.pm             |  4 ++--
src/include/access/xlog.h                     |  4 ++--
src/test/recovery/t/019_replslot_limit.pl     | 10 ++++-----
12 files changed, 58 insertions(+), 48 deletions(-)