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

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