oauth: Don't log discovery connections by default

oauth: Don't log discovery connections by default

Currently, when the client sends a parameter discovery request within
OAUTHBEARER, the server logs the attempt with

    FATAL:  OAuth bearer authentication failed for user

These log entries are difficult to distinguish from true authentication
failures, and by default, libpq sends a discovery request as part of
every OAuth connection, making them annoyingly noisy. Use the new
PG_SASL_EXCHANGE_ABANDONED status to suppress them.

Patch by Zsolt Parragi, with some additional comments added by me.

Author: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAN4CZFPim7hUiyb7daNKQPSZ8CvQRBGkVhbvED7yZi8VktSn4Q%40mail.gmail.com