iterator() return type in Cache

[dhruvv90]

Proposal : to change return type of iterator() method in class Cache from Iterator<Cache.Entry<K, V>> to Iterator<? extends Cache.Entry<K, V>>

This will help JSR implementations to create Iterables of custom types, since Generics are not covariant.
There might be reasons this wasn't done this way, but sorry i'm not aware of that if that's the case