pgsql_fdw in contrib

I'd like to propose pgsql_fdw, FDW for PostgreSQL, as a contrib module
in core, again.  This patch is basically rebased version of the patches
proposed in 9.2 development cycle, and contains some additional changes
such as concern about volatile variables for PG_CATCH blocks.  In
addition, I combined old three patches (pgsql_fdw core functionality,
push-down support, and analyze support) into one patch for ease of
review. (I think these features are necessary for pgsql_fdw use.)

After the development for 9.2 cycle, pgsql_fdw can gather statistics
from remote side by providing sampling function to analyze module in
backend core, use them to estimate selectivity of WHERE clauses which
will be pushed-down, and retrieve query results from remote side with
custom row processor which prevents memory exhaustion from huge result set.

It would be possible to add some more features, such as ORDER BY
push-down with index information support, without changing existing
APIs, but at first add relatively simple pgsql_fdw and enhance it seems
better.  In addition, once pgsql_fdw has been merged, it would help
implementing proof-of-concept of SQL/MED-related features.

Regards,
--
Shigeru HANADA