Some update for JSegcache

[JunchengYTwitter]

This PR contains multiple things (let me know if you want to split them)

1. Add get_age in segments, then add a new field age in Item so that we can return age to caller.
2. Add RichItem to return item_info and item_info_ptr, these two are used to support opportunistic concurrency control. Not sure adding a new struct is better than adding the fields to Item, but given there might be fields (like these) that we often do not need, creating a new struct might be better.
3. add a call to expire before we evict, this avoids the problem that users may forget to call expire.
4. add key size check when inserting to the cache.

[CLAassistant]

All committers have signed the CLA.

[brayniac]

@JunchengYTwitter - this looks good overall. Just needs rustfmt. I'm okay with this remaining a single PR.

[JunchengYTwitter]

updated