Provide file modification date on `Entry`

[Kaliumhexacyanoferrat]

For some operations such as eTag generation, frameworks typically use the modification date of the file being served. Providing the date on Entry would remove an additional operation on the framework.

[Kaliumhexacyanoferrat]

Okay, meta data is not re-evaluated so it will only be computed once when filling the cache. So maybe it would make more sense to add a Refresh() call that does a single fstat() syscall and updates the fields. As we already have the handle this should be fairly cheap. For a highly concurrent scenario we might want to limit the updates to every 100ms or so? It would be up to the framework to decide whether assets are immutable so the meta data does not need to be updated or not.