[Feature] Use MetaStore/EntityStore native expiration

[rykov]

Is there any interest in optionally passing 'max-age' to the underlying datastore on write to automatically expire stale data? It has potential for memory saving if long-tail entries get cleaned up before waiting for a subsequent request to be invalidated & replaced.

[Mange]

This is almost a necessity. We just realized that our cache machines became full from requests that happened a long time ago and has never been opened again. If the server pass "This page expires on the following date", the underlying store should exorcise it when the time comes.

We're still searching for a way to fix our machines again. :-(