Native support for conditional iteration

Ported from: https://github.com/GoogleCloudPlatform/DataflowJavaSDK/issues/50

There are a variety of use cases which would benefit from native support for conditional iteration.

For instance, http://stackoverflow.com/questions/31654421/conditional-iterations-in-google-cloud-dataflow/31659923?noredirect=1#comment51264604_31659923 asks about being able to write a loop like the following:

```

PCollection data = ...
while(needsMoreWork(data)) {
data = doAStep(data)
}

```

If there are specific use cases please let us know the details. In the future we will use this issue to post progress updates.

Imported from Jira [BEAM-106](https://issues.apache.org/jira/browse/BEAM-106). Original Jira may contain additional context.
Reported by: lcwik.

No source files, tests, or entry points are named. Start by reviewing the imported Jira issue and linked Stack Overflow discussion, then identify concrete conditional-iteration use cases and the required Beam semantics. The issue is done only when a defined native iteration design is implemented with coverage for its agreed behavior.

Written by the indexing model from the issue text.