GetWithExpirationUpdate - atomic implementation

[paddlesteamer]

This PR is fixed version of #96. Main changes are:

• Type of cache.items are converted from map[string]Item to map[string]*Item. I needed to do it because, in GetWithExpirationUpdate, it is the only way to modify the Expiration field of an Item. The other way around (re-setting the item) needs a write lock, therefore blocks all reads/writes to items. Not convenient for 'cache-get's.
• Now every Item has its own RWLock. This way, we don't need a write lock in GetWithExpirationUpdate.