simplify generics

this allows us to use `Points<Rollup> input` in places where we were using `Points<? extends Rollup> input`.
Further streamline use of generics wrt `Rollup`.

Rollup type is responsible for calling the right method. This replaces the `instanceof` chains in `Rollup.java`.

If we want to take this further, `SimpleNumber` could be an instance of `Rollup` whose operations are basically identity.

Then we could get the type erasure out of `AstyanaxReader.getDataToRoll()`.  (would return `Points<Rollup>`.
Fully parameterized AstyanaxReader.getDataToRoll().
merge from upstream. Fixed small confluxed in HistogramRollupTest.java.
* Simplify the Rollup.Type interface to one method.
* Get it out of RollupContext.
Fix accidental erasure in AstyanaxReader.getDataToRoll().  Unfortunately, this could only be done by having one method per return type.
restore disabled test.
put broken test back together.
Merge branch 'bf_add_histogram_support' into generics_changes_for_metric_types
Rename AstyanaxReader.getBasicDataToRoll() -> .getBasicRollupDataToRoll()
Merge pull request #102 from gdusbabek/generics_changes_for_metric_types

Generics changes for metric types