Support helper ttl= annotations for Basic authentication

[yadij]

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... and clean up the TTL / expiry information for all authentication schemes behind an API of Auth::User.

Fixes a small bug where Basic auth credentials could be expired one second earlier than they should have been.

Fixes a small bug where Basic auth credentials could be expired by authenticate_ttl earlier than their squid.conf credentialsttl is set to. These two configuration settings are supposed to cover different time periods in credentials lifetime within Squid memory.

Documentation updated to clarify the various credential TTL config setting meaning and behaviour impact.

[yadij]

@rousskov, ball is in your court now. afaics lots of review items to sign off on your part and two discussions yet to reach conclusion.