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