pgsql: Added async query capability.

Log Message:
-----------
Added async query capability. Original patch by
Kai Londenberg, modified by Joe Conway

Modified Files:
--------------
    pgsql/contrib/dblink:
        README.dblink (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/README.dblink.diff?r1=1.13&r2=1.14)
        dblink.c (r1.57 -> r1.58)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c.diff?r1=1.57&r2=1.58)
        dblink.h (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.h.diff?r1=1.16&r2=1.17)
        dblink.sql.in (r1.10 -> r1.11)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.sql.in.diff?r1=1.10&r2=1.11)
    pgsql/contrib/dblink/doc:
        misc (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/doc/misc.diff?r1=1.3&r2=1.4)
        query (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/doc/query.diff?r1=1.4&r2=1.5)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out.diff?r1=1.17&r2=1.18)
    pgsql/contrib/dblink/sql:
        dblink.sql (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/sql/dblink.sql.diff?r1=1.15&r2=1.16)