Added local strategy SORT to DataSinkTask and extended DataSinkTaskTest

Fixes #146 and adds support for locally sorted output. Globally sorted output is not supported yet.
This pull request only adds the local strategy SORT to the DataSinkTask and extends the DataSinkTaskTest.
The Pact optimizer had already set all required task configurations.

@StephanEwen Please check that the canceling logic and tests are OK.