Preaggreated Metrics and Rollups

This PR sets us up to introduce a statsd module.

* Change the contract for getting a reference to a column family to require granularity only. 
  * This required changing the plumbing in RollupContext and RollupRunnable to not include references to column families, but to derive them from granularity when needed.
  * In the future, column families will be derived from granularity + stat type.
* Expose the preaggregated metrics in AstyanaxIO.
* Introduce preaggregated metric and rollup types.
* AstyanaxReader
  * getAllMetadata method
  * generic getDataToRoll method.
* AstyanaxWriter
  * generic insertMetrics method
* Favor `Rollup` instances in reader/writer methods