Rewrite dblink_record_internal() and dblink_fetch() to use a tuplestore

Rewrite dblink_record_internal() and dblink_fetch() to use a tuplestore
(SFRM_Materialize mode) to return tuples. Since we don't return from the
dblink function in tuplestore mode, release the PGresult with a PG_CATCH
block on error. Also rearrange to share the same code to materialize the
tuplestore. Patch by Takahiro Itagaki.