[REEF-3]Create basic Task scheduler as an example

The current Task scheduler example only supports one evaluator
at a time. This pull request addresses this issue to run multiple
evaluators simultaneously and introduce a new REST interface to
dynamically set the maximum number of evaluator.

* The new API : `/reef-example-scheduler/v1/max-eval?num={num}`.
* Quite a lot of code rearrange and refactoring are done.

JIRA: [REEF-3](https://issues.apache.org/jira/browse/REEF-3)
when methods are public, they could still be referenced by out-of-package places that get a hold of a reference of an object of Type TaskEntity via one of the implemented interfaces. As there are none here, it really doesn't make a difference.