pgsql: Morph pg_replication_slots.min_safe_lsn to safe_wal_size

Morph pg_replication_slots.min_safe_lsn to safe_wal_size

The previous definition of the column was almost universally disliked,
so provide this updated definition which is more useful for monitoring
purposes: a large positive value is good, while zero or a negative value
means danger. This should be operationally more convenient.

Backpatch to 13, where the new column to pg_replication_slots (and the
feature it represents) were added.

Author: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reported-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Discussion: https://postgr.es/m/9ddfbf8c-2f67-904d-44ed-cf8bc5916228@oss.nttdata.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c54b5891f415df36809de1aeb97e4574d5456d69

Modified Files
--------------
doc/src/sgml/catalogs.sgml | 7 ++++--
src/backend/access/transam/xlog.c | 6 ++---
src/backend/catalog/system_views.sql | 2 +-
src/backend/replication/slotfuncs.c | 40 +++++++++++++++++++++++--------
src/include/access/xlog_internal.h | 7 ++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 4 ++--
src/test/recovery/t/019_replslot_limit.pl | 22 ++++++++---------
src/test/regress/expected/rules.out | 4 ++--
9 files changed, 60 insertions(+), 34 deletions(-)