Caching in the Tycho VR

As part of the Tycho benchmarking effort I have added caching to the VR, this completes the existing list of new stuff to write.

The new CacheManager receives a copy of any remote results from the VR. The CacheManager uses the same LocalStore as the VR which means cached records are automatically retrieved as part of normal client queries. A separate Invalidator thread is used to manage the cached records, currently the implementation uses TTL although we could have more advanced functionality such as leasing.