pgsql: Make dblink interruptible, via new libpqsrv APIs.

Make dblink interruptible, via new libpqsrv APIs.

This replaces dblink's blocking libpq calls, allowing cancellation and
allowing DROP DATABASE (of a database not involved in the query). Apart
from explicit dblink_cancel_query() calls, dblink still doesn't cancel
the remote side. The replacement for the blocking calls consists of
new, general-purpose query execution wrappers in the libpqsrv facility.
Out-of-tree extensions should adopt these. Use them in postgres_fdw,
replacing a local implementation from which the libpqsrv implementation
derives. This is a bug fix for dblink. Code inspection identified the
bug at least thirteen years ago, but user complaints have not appeared.
Hence, no back-patch for now.

Discussion: https://postgr.es/m/20231122012945.74@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d3c5f37dd543498cc7c678815d3921823beec9e9

Modified Files
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contrib/dblink/dblink.c | 28 +++--
contrib/postgres_fdw/connection.c | 88 +++-----------
contrib/postgres_fdw/deparse.c | 2 +-
contrib/postgres_fdw/postgres_fdw.c | 10 +-
contrib/postgres_fdw/postgres_fdw.h | 2 +-
doc/src/sgml/dblink.sgml | 9 ++
.../libpqwalreceiver/libpqwalreceiver.c | 9 +-
src/include/libpq/libpq-be-fe-helpers.h | 127 +++++++++++++++++++++
8 files changed, 180 insertions(+), 95 deletions(-)