pgsql: dblink/isolationtester/fe_utils: Use new cancel API

dblink/isolationtester/fe_utils: Use new cancel API

Commit 61461a300c1c introduced new functions to libpq for cancelling
queries.  This replaces the usage of the old ones in parts of the
codebase with these newer ones.  This specifically leaves out changes to
psql and pgbench, as those would need a much larger refactor to be able
to call them due to the new functions not being signal-safe; and also
postgres_fdw, because the original code there is not clear to me
(Álvaro) and not fully tested.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQT_VgOWWENUqvUV9xQmbaCyXjtRRAYO8W07oqashk_N+g@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/66ab9371a23320cf608e68e8e5d2811992941bea

Modified Files
--------------
contrib/dblink/dblink.c              | 26 ++++++++++++++++----------
src/fe_utils/connect_utils.c         | 11 +++--------
src/test/isolation/isolationtester.c | 29 ++++++++++++-----------------
3 files changed, 31 insertions(+), 35 deletions(-)