contrib: auth_delay module

The attached patch is a contrib module to set login restrictions on users with
too many authentication failure. The administrator could manage several GUC
parameters to control the login restrictions which are listed below.
- set the wait time when password authentication fails.
- allow the wait time grows when users of the same IP consecutively logon failed.
- set the maximum authentication failure number from the same user. The system
will prevent a user who gets too many authentication failures from entering the
database.

I hope this will be useful to future development.
Thanks.
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zhcheng(at)ceresdata(dot)com