pgsql: Add temporary debug info to help debug 019_replslot_limit.pl fai

Add temporary debug info to help debug 019_replslot_limit.pl failures.

I have not been able to reproduce the occasional failures of
019_replslot_limit.pl we are seeing in the buildfarm and not for lack of
trying. The additional logging and increased log level will hopefully help.

Will be reverted once the cause is identified.

Discussion: https://postgr.es/m/20220218231415.c4plkp4i3reqcwip@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/afdeff10526e29e3fc63b18c08100458780489d9

Modified Files
--------------
src/backend/replication/slot.c            | 21 +++++++++++++++++++++
src/bin/pg_basebackup/pg_basebackup.c     | 10 +++++++++-
src/test/recovery/t/019_replslot_limit.pl |  5 ++++-
3 files changed, 34 insertions(+), 2 deletions(-)