Port to PostgreSQL 9.2+ new FDW API

The Foreign Data Wrapper API changed a lot since PG 9.2, requiring
us to provide more callbacks to help the query planner in choosing
a good query plan (esp. since we can now provide several pathes).

Those new callbacks now replaces the generic PlanForeignScan.

See http://www.postgresql.org/docs/devel/static/release-9-2.html

And this Tom Lane commit :
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=b14953932dfdda7d915b9e276a09df8458efeec8