Improve wording of log messages triggered by max_slot_wal_keep_size.

Improve wording of log messages triggered by max_slot_wal_keep_size.

The one about "terminating process to release replication slot" told
you nothing about why that was happening.  The one about "invalidating
slot because its restart_lsn exceeds max_slot_wal_keep_size" told you
what was happening, but violated our message style guideline about
keeping the primary message short.  Add DETAIL/HINT lines to carry
the appropriate detail and make the two cases more uniform.

While here, fix bogus test logic in 019_replslot_limit.pl: if it timed
out without seeing the expected log message, no test failure would be
reported.  This is flat broken since commit 549ec201d removed the test
counts; even before that it was horribly bad style, since you'd only
get told that not all tests had been run.

Kyotaro Horiguchi, reviewed by Bertrand Drouvot; test fixes by me

Discussion: https://postgr.es/m/20211214.130456.2233153190058148084.horikyota.ntt@gmail.com