MutableTTLCache.hit should call get(k) instead of backingCache.get

[aschwager]

When implementing a ReadableStore.withCache(someStore, MutableTTLCache) I noticed that entries were not properly expiring and traced the issue back to this line. Calling hit(k) on the MutableTTLCache seems to call directly into the backing cache (in this case a JLinkedHashMap) and does not apply any of the ttl-ness filtering found in MutableTTLCache.get. Hit then calls +=, which adds the entry back to the cache with a new TTL