dblink query interruptibility

=== Background

Something as simple as the following doesn't respond to cancellation.  In
v15+, any DROP DATABASE will hang as long as it's running:

  SELECT dblink_exec(
    $$dbname='$$||current_database()||$$' port=$$||current_setting('port'),
    'SELECT pg_sleep(15)');

https://postgr.es/m/4B584C99.8090004@enterprisedb.com proposed a fix back in
2010.  Latches and the libpqsrv facility have changed the server programming
environment since that patch.  The problem statement also came up here:

On Thu, Dec 08, 2022 at 06:08:15PM -0800, Andres Freund wrote:
> dblink.c uses a lot of other blocking libpq functions, which obviously also
> isn't ok.


=== Key decisions

This patch adds to libpqsrv facility.  It dutifully follows the existing
naming scheme.  For greppability, I am favoring renaming new and old functions
such that the libpq name is a substring of this facility's name.  That is,
rename libpqsrv_disconnect to srvPQfinish or maybe libpqsrv_PQfinish().  Now
is better than later, while pgxn contains no references to libpqsrv.  Does
anyone else have a preference between naming schemes?  If nobody does, I'll
keep today's libpqsrv_disconnect() style.

I was tempted to add a timeout argument to each libpqsrv function, which would
allow libpqsrv_get_result_last() to replace pgfdw_get_cleanup_result().  We
can always add a timeout-accepting function later and make this thread's
function name a thin wrapper around it.  Does anyone feel a mandatory timeout
argument, accepting -1 for no timeout, would be the right thing?


=== Minor topics

It would be nice to replace libpqrcv_PQgetResult() and friends with the new
functions.  I refrained since they use ProcessWalRcvInterrupts(), not
CHECK_FOR_INTERRUPTS().  Since walreceiver already reaches
CHECK_FOR_INTERRUPTS() via libpqsrv_connect_params(), things might just work.

This patch contains a PQexecParams() wrapper, called nowhere in
postgresql.git.  It's inessential, but twelve pgxn modules do mention
PQexecParams.  Just one mentions PQexecPrepared, and none mention PQdescribe*.

The patch makes postgres_fdw adopt its functions, as part of confirming the
functions are general enough.  postgres_fdw create_cursor() has been passing
the "DECLARE CURSOR FOR inner_query" string for some error messages and just
inner_query for others.  I almost standardized on the longer one, but the test
suite checks that.  Hence, I standardized on just inner_query.

I wrote this because pglogical needs these functions to cooperate with v15+
DROP DATABASE (https://github.com/2ndQuadrant/pglogical/issues/418).

Thanks,
nm