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Storage is now done through a StorageAdapter interface, which specifies
how to save and load expirations. There are two adapters provided: one
that saves nowhere and one that saves in a gob file (to mimic the
original behaviour of this package.)

Code using the old expiration store will automatically migrate to the
new store format on load/save.

This avoids a potential infinite recursion when an object that is
destroyed calls back to have itself de-registered from expiration.