An entry is expired when it hasn't been used for a while, but the user
of the cache may know it is still doing something in the background.
SetCanExpire sets a function which is asked before expiring an entry
and can keep it in the cache until it is ready to go.
The function is called without the cache locked as it may take a
while, so an entry which is used in the meantime isn't expired.
(cherry picked from commit 49d8566ec5)
Nick Craig-Wood
2f84013eec