Latency constraint fluent API with two edges

[derSascha]

The API supports our twitter-sentiment-job only with a workaround mapper.

The following does not work:

DataStream stream1 = source
    .beginLatencyConstraint("one", 123)
    ....
    .finishLatencyConstraint();

DataStream stream2 = source
    .beginLatencyConstraint("two", 123)
    ....
    .finishLatencyConstraint();

In words: We have two constraints on two different paths from our source.

TODO:

• [ ] Support a way to define constraint on one output path.
  • Variant 1: Use next defined path (next operator after beginLatencyConstraint defines the path)
  • Variant 2: Add some parameter (like nepehle version)
• [ ] Same problem happens after finishLatencyConstraint (two possible paths).
  • Possible solution: Don't add the edge, simply finish the constraint on the vertex before .fini... call.
• [ ] Not so important: Calling .finish without a name after calling .begin with a name feels confusing.
  • Possible solution: Have both Variants. A parameter less and one version with identifier validation (check if given id matches current constraint).