Use WaitLatch for {pre, post}_auth_delay instead of pg_usleep

Hi,

As suggested in [1], starting a new thread for discussing $subject
separately. {pre, post}_auth_delay waiting  logic currently uses
pg_usleep which can't detect postmaster death. So, there are chances
that some of the backends still stay in the system even when a
postmaster crashes (for whatever reasons it may be). Please have a
look at the attached patch that does $subject. I pulled out some of
the comments from the other thread related to the $subject, [2], [3],
[4], [5].

[1] - https://www.postgresql.org/message-id/YOv8Yxd5zrbr3k%2BH%40paquier.xyz
[2] - https://www.postgresql.org/message-id/162764.1624892517%40sss.pgh.pa.us
[3] - https://www.postgresql.org/message-id/20210705.145251.462698229911576780.horikyota.ntt%40gmail.com
[4] - https://www.postgresql.org/message-id/flat/20210705155553.GD20766%40tamriel.snowman.net
[5] - https://www.postgresql.org/message-id/YOOnlP4NtWVzfsyb%40paquier.xyz

Regards,
Bharath Rupireddy.