FDW-based dblink (WIP)

Here is a WIP patch for a foreign data wrapper based dblink.

It integrates dblink module into core and adds a new functionality,
automatic transaction management. The new interface of dblink is
exported by include/foreign/dblink.h. We can easily write a connector
module for another database because we can reuse transaction and
resource management parts in core.


Syntax to create FDW with connector is below:
    CREATE FOREIGN DATA WRAPPER postgresql
        VALIDATOR postgresql_fdw_validator
        CONNECTOR postgresql_fdw_connector
        OPTIONS (...);

contrib/dblink2 is a sample of postgres connector. It exports one function:
    CREATE FUNCTION postgresql_fdw_connector(options internal)
      RETURNS internal -- returns a connection object

Basic dblink functions are moved to postgres core:
            Name       | Result type  | Argument data types
    -------------------+--------------+----------------------
     dblink            | SETOF record | text, text
     dblink_close      | boolean      | integer
     dblink_connect    | boolean      | text
     dblink_connect    | boolean      | text, text
     dblink_disconnect | boolean      | text
     dblink_exec       | bigint       | text, text
     dblink_fetch      | SETOF record | integer, integer
     dblink_open       | integer      | text, text

The new dblink can work together closely with local transactions. If a
local transaction is committed or rollbacked, remote transactions take
the same status with the local one. Please set max_prepared_transactions
to 1 or greater if you could test the patch.

I want pretty much the automatic transaction management. It is useful to
write applied modules like materialized-view-over-network. But it should
be able to be turned off if we don't want it. I'll work on those parts next.

    -- connect
    CREATE SERVER server_postgres FOREIGN DATA WRAPPER postgresql;
    SELECT dblink_connect('conn_postgres', 'server_postgres');
    -- commit both local and remote transactions.
    BEGIN;
    SELECT dblink_exec('conn_postgres', 'UPDATE ...');
    COMMIT;
    -- rollback both local and remote transactions.
    BEGIN;
    SELECT dblink_exec('conn_postgres', 'UPDATE ...');
    ROLLBACK;
    -- disconnect
    SELECT dblink_disconnect('conn_postgres');

I've not ported all features in present dblink, but I'd like to hear
wheather the goal and the concepts are reasonable. Comments welcome.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center