Get rid of `Rollup.getCount()` method.

It has been pushed down into `IBasicRollup`.
Demote `Rollup.java` to an interface.
The rest of the `*Rollup` implementations and rollup operations.

* `CounterRollup`, `GaugeRollup` and `SetRollup` all extend `SingleValueRollup`
* Tests verify rollup semantics
* No rollup tests for `SetRollup` since they will never be rolled up.
CounterRollup, SetRollup, GaugeRollup serialization and tests

* We only serialize a single value for each type
* A single serializer is used for each and uses the type byte to determine what to return.