[PATCH] Exponential backoff for auth_delay

Hi,

we had a conversation with a customer about security compliance a while
ago and one thing they were concerned about was avoiding brute-force
attemps for remote password guessing. This is should not be a big
concern if reasonably secure passwords are used and increasing SCRAM
iteration count can also help, but generally auth_delay is recommended
for this if there are concerns.

This patch adds exponential backoff so that one can choose a small
initial value which gets doubled for each failed authentication attempt
until a maximum wait time (which is 10s by default, but can be disabled
if so desired).

Currently, this patch tracks remote hosts but not users, the idea being
that a remote attacker likely tries several users from a particular
host, but this could in theory be extended to users if there are
concerns.

The patch is partly based on an earlier, more ambitious attempt at
extending auth_delay by 成之焕 from a year ago:
https://postgr.es/m/AHwAxACqIwIVOEhs5YejpqoG.1.1668569845751.Hmail.zhcheng@ceresdata.com

Michael