Prevent eager deletion of invalid persistent session cookie

[danschultzer]

Resolves #356

This prevent eager deletion of cookie. The cookie expiration date is set to 10 seconds in the future rather than 1 second in the past, so the cookie will automatically expires 10 seconds after the last request. This should give enough time in case there are a multitude of requests as described in #356 while rolling the persistent session.

I've also added the :persistent_session_cookie_opts to the cookie deletion and expiration.

[liamwhite]

Could the time drift be configurable? Context: I have some users who use very high-latency internet connections, and I'm wondering if higher values for time drift would make more sense there.

[danschultzer]

Added :persistent_session_cookie_expiration_drift, but 10 seconds should be enough. High latency may actually be preferable since the problem is when the requests comes in quicker than the GenServer can finish the async update.

[danschultzer]

Renamed it to :persistent_session_cookie_expiration_timeout, I think drift doesn't really make sense.