[PATCH] postgres_fdw extension support

Hi all,

Attached is a patch that implements the extension support discussed at
PgCon this year during the FDW unconference sesssion. Highlights:

* Pass extension operators and functions to the foreign server
* Only send ops/funcs if the foreign server is declared to support the
relevant extension, for example:

CREATE SERVER foreign_server
        FOREIGN DATA WRAPPER postgres_fdw
        OPTIONS (host '127.0.0.1', port '5432', dbname 'my_db',
extensions 'cube, seg');

Github branch is here:
  https://github.com/pramsey/postgres/tree/fdw-extension-suppport

Synthetic pull request for easy browsing/commenting is here:
  https://github.com/pramsey/postgres/pull/1

Thanks!

Paul