pgsql: Improve efficiency of dblink by using libpq's new row processor

Improve efficiency of dblink by using libpq's new row processor API.

This patch provides a test case for libpq's row processor API.
contrib/dblink can deal with very large result sets by dumping them into
a tuplestore (which can spill to disk) --- but until now, the intermediate
storage of the query result in a PGresult meant memory bloat for any large
result. Now we use a row processor to convert the data to tuple form and
dump it directly into the tuplestore.

A limitation is that this only works for plain dblink() queries, not
dblink_send_query() followed by dblink_get_result(). In the latter
case we don't know the desired tuple rowtype soon enough. While hack
solutions to that are possible, a different user-level API would
probably be a better answer.

Kyotaro Horiguchi, reviewed by Marko Kreen and Tom Lane

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6f922ef88e43b3084cdddf4b5ffe525a00896a90

Modified Files
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contrib/dblink/dblink.c | 421 ++++++++++++++++++++++++++++++++++++++--------
doc/src/sgml/dblink.sgml | 20 ++-
2 files changed, 366 insertions(+), 75 deletions(-)