client: Further datapoint retrieval work.
client: Further datapoint retrieval work.

- Only perform a refresh on points that are due for refresh.
- Store a reference to the point class so that other client
  implementations can supply their own compatible HaystackPoint
  subclass.

ChristianTremblay/pyhaystack,vrtsystems/pyhaystack,ChristianTremblay/pyhaystack