pgsql: Set pg_class.reltuples for partitioned tables

Set pg_class.reltuples for partitioned tables

When commit 0827e8af70f4 added auto-analyze support for partitioned
tables, it included code to obtain reltuples for the partitioned table
as a number of catalog accesses to read pg_class.reltuples for each
partition. That's not only very inefficient, but also problematic
because autovacuum doesn't hold any locks on any of those tables -- and
doesn't want to. Replace that code with a read of pg_class.reltuples
for the partitioned table, and make sure ANALYZE and TRUNCATE properly
maintain that value.

I found no code that would be affected by the change of relpages from
zero to non-zero for partitioned tables, and no other code that should
be maintaining it, but if there is, hopefully it'll be an easy fix.

Per buildfarm.

Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Zhihong Yu <zyu(at)yugabyte(dot)com>
Discussion: https://postgr.es/m/1823909.1617862590@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0e69f705cc1a3df273b38c9883fb5765991e04fe

Modified Files
--------------
src/backend/commands/analyze.c | 12 ++++++++++
src/backend/commands/tablecmds.c | 47 ++++++++++++++++++++++++++++++++++++-
src/backend/postmaster/autovacuum.c | 39 +-----------------------------
3 files changed, 59 insertions(+), 39 deletions(-)