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 ae9bfc5d
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