Implement L2 cache

[mrsarm]

Currently there is a L1 cache that allows recover the resource from the cache, but if the time expires, reqclient goes to the server to get a fresh value blocking the request until the response (but asynchronous, of course ;) .

The L2 cache could never expire, or have a higher TTL, and the most important, it will allow to reqclient return a value and fire the request to get a fresh value for the next call (the value obtained from the server will update both caches).