idea: reload cache when its about to expire in __destruct()

[Jimbolino]

Currently when the cache is expired (every 15 sec), unleash needs to repopulate it.
In our case this causes a 200-300 ms delay on the total request.
Also on high traffic websites its possible that multiple users are making the same request at (almost) the same time.

I've tested with a simple mechanism that sets an "expires_at" timestamp, and a few seconds before the timestamp is reached it will refresh the cache with new data.
Because the refresh happens in the __destruct(), the user that triggers the refresh does not notice any delays.

Would you accept a PR for such a mechanism?

[mikefrancis]

@Jimbolino Sorry about the delay, feel free to submit a PR 👍