Concurrency: parallel blocking Sink

[eledhwen]

It should be possible to run Sinks in a parallel fashion while still requiring them to be completed before exiting the pipeline.

Current implementation makes a distinction between serial synchronous and parallel asynchronous ; instead we most often want parallel synchronous and parallel asynchronous.
This can be straightforward to implement using the Future API.

It should be noted however that this requires additional attention to the ServiceExecutor: making most/all Sink parallel requires appropriate sizing, and possibly thread pooling between pipelines. We can already make such adjustments via the setSinkExecutor and setSinkExecutorProvider methods on the pipeline builders, but it may need to be better integrated.