pgsql: Re-establish postgres_fdw connections after server or user mappi

Re-establish postgres_fdw connections after server or user mapping changes.

Previously, postgres_fdw would keep on using an existing connection even
if the user did ALTER SERVER or ALTER USER MAPPING commands that should
affect connection parameters. Teach it to watch for catcache invals
on these catalogs and re-establish connections when the relevant catalog
entries change. Per bug #14738 from Michal Lis.

In passing, clean up some rather crufty decisions in commit ae9bfc5d6
about where fields of ConnCacheEntry should be reset. We now reset
all the fields whenever we open a new connection.

Kyotaro Horiguchi, reviewed by Ashutosh Bapat and myself.
Back-patch to 9.3 where postgres_fdw appeared.

Discussion: https://postgr.es/m/20170710113917.7727.10247@wrigleys.postgresql.org

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cb2b1cac0c331493dc3a316f92834bfad1b62149

Modified Files
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contrib/postgres_fdw/connection.c | 131 +++++++++++++++++++++----
contrib/postgres_fdw/expected/postgres_fdw.out | 37 +++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 20 ++++
3 files changed, 170 insertions(+), 18 deletions(-)