pgsql: Stats: use schemaname/relname instead of regclass.

Stats: use schemaname/relname instead of regclass.

For import and export, use schemaname/relname rather than
regclass.

This is more natural during export, fits with the other arguments
better, and it gives better control over error handling in case we
need to downgrade more errors to warnings.

Also, use text for the argument types for schemaname, relname, and
attname so that casts to "name" are not required.

Author: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Discussion: https://postgr.es/m/CADkLM=ceOSsx_=oe73QQ-BxUFR2Cwqum7-UP_fPe22DBY0NerA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/650ab8aaf1957863ae14c80265e79f5d903b49fd

Modified Files
--------------
doc/src/sgml/func.sgml | 45 ++--
src/backend/statistics/attribute_stats.c | 61 ++++--
src/backend/statistics/relation_stats.c | 54 +++--
src/backend/statistics/stat_utils.c | 22 ++
src/bin/pg_dump/pg_dump.c | 25 ++-
src/bin/pg_dump/t/002_pg_dump.pl | 6 +-
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 8 +-
src/include/statistics/stat_utils.h | 2 +
src/test/regress/expected/stats_import.out | 337 ++++++++++++++++++++---------
src/test/regress/sql/stats_import.sql | 306 +++++++++++++++++---------
11 files changed, 579 insertions(+), 289 deletions(-)