pgsql: Arrange to cache FdwRoutine structs in foreign tables' relcache

Arrange to cache FdwRoutine structs in foreign tables' relcache entries.

This saves several catalog lookups per reference. It's not all that
exciting right now, because we'd managed to minimize the number of places
that need to fetch the data; but the upcoming writable-foreign-tables patch
needs this info in a lot more places.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1908abc4a37d397356c9cdf0fd31c33a86281d63

Modified Files
--------------
src/backend/commands/analyze.c | 2 +-
src/backend/executor/nodeForeignscan.c | 2 +-
src/backend/foreign/foreign.c | 46 ++++++++++++++++++++++++++++++++
src/backend/optimizer/path/allpaths.c | 3 --
src/backend/optimizer/util/plancat.c | 8 +++++
src/backend/utils/cache/relcache.c | 5 +++-
src/include/foreign/fdwapi.h | 1 +
src/include/utils/rel.h | 11 +++++++
8 files changed, 72 insertions(+), 6 deletions(-)