adding support for scram authentication in health_check and sr_check connections.

adding support for scram authentication in health_check and sr_check connections.

explicit backend connections in Pgpool-II uses make_persistent_db_connection(),
which use a different code path from frontend connection to authenticating with
the backend and requires its own SCRAM auth handling.
(reported by Jesper Pedersen <jesper.pedersen@redhat.com>)

Commit also fixes a problem in md5 and scram frontend authentication, where
sometimes a client was able to connect without the password.

Finally the commit also allows to connect to Pgpool-II using trust authentication
with pgpool-II even when backends requires some different authentication method.