add GetWithExpirationUpdate

[sbabiv]

In many cases, it is convenient to get items with an updated cache lifetime.

• GetWithExpirationUpdate - returns item and updates its cache expiration time

[mrbenosborne]

Hi @sbabiv,

This repository seems to have gone stale and I am going to keep the go-cache running, I have added your PR if you are interested.

https://github.com/mrbenosborne/go-cache/pull/1

[paddlesteamer]

I think it is problematic. I'm worried about this (edge) case:

1. we get the item, check the expiration, it is almost expired but not expired yet
2. we do read unlock
3. onEvicted wins the race condition and do something because it thinks the item is expired
4. then we do write lock and reset the item

so it's not atomic. It may cause problems when it's used with user-provided onEvicted function.

I think to complete this completely the items should have their own mutexes and the changes inside of the items (like change in Expiration) shouldn't be considered in cache's write lock scope.