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
Merge branch 'generics_changes_for_metric_types' into statsd_with_histos
* convert `Stat` instances to `Metric` instances.
* figure out a heuristic for detected internal vs external metrics and obtaining a tenantId
* store stat types in a cache.
Rename AstyanaxReader.getBasicDataToRoll() -> .getBasicRollupDataToRoll()
Merge branch 'generics_changes_for_metric_types' into statsd_with_histos
Catch-all function to set the value of an `AbstractRollupStat`.
bug fix: Variance.toString() should trigger a compute if necessary.
checking in the rackspace agent plugin script
added exception and extra logging for bf-rollups
Merge pull request #100 from wolfdancer/rackspace_agent_script

checking in the rackspace agent plugin script
Merge pull request #102 from gdusbabek/generics_changes_for_metric_types

Generics changes for metric types
Introduce TimerRollup and some tests.
pre-aggregated column family.