A PrefetchingTermLookup is needed to provide labels for formatting statements in the entity view.
This ticket is mainly about creating the service, and it will be wired up seperately.

This should:

• Only work for labels.
• descriptions and aliases should throw some kind of exception ( maybe a BadMethodCallException )
• Prefetch terms using batched calls to wbgetentities
  • max entities that can be requested in one call should be assumed to be 50 for now
  • Do not fetch more than is needed (only fetch labels, not statements and descriptions etc)