Suggestion for GetWithExpiration method for expirable cache

[ardaasln]

I think I have a use case for this.

In a highly concurrent traffic scenario, when an item is expired, many requests fail to get the value and hence, they flood the other data source (a db in my case). I am currently using https://github.com/patrickmn/go-cache and it has GetWithExpiration method so that before a minute when expiration occurs, a goroutine singlehandedly re-populates the cache (with the help of TryLock).

I might work on this if this sounds valuable.