pgsql: Add pg_stat_autovacuum_scores system view.

Add pg_stat_autovacuum_scores system view.

This view contains one row for each table in the current database,
showing the current autovacuum scores for that specific table. It
also shows whether autovacuum would vacuum or analyze the table.

Bumps catversion.

Author: Sami Imseih <samimseih(at)gmail(dot)com>
Reviewed-by: Satyanarayana Narlapuram <satyanarlapuram(at)gmail(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Robert Treat <rob(at)xzilla(dot)net>
Discussion: https://postgr.es/m/CAA5RZ0s4xjMrB-VAnLccC7kY8d0-4806-Lsac-czJsdA1LXtAw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/87f61f0c82806b7e4201f15bd77920e9e7108b11

Modified Files
--------------
doc/src/sgml/maintenance.sgml | 6 ++
doc/src/sgml/monitoring.sgml | 179 +++++++++++++++++++++++++++++++++++
src/backend/catalog/system_views.sql | 18 ++++
src/backend/postmaster/autovacuum.c | 73 ++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 7 ++
src/test/regress/expected/rules.out | 15 +++
7 files changed, 299 insertions(+), 1 deletion(-)