Abstract interface for a "sink" accepting multiple entire streams.
A consumer can accept a number of consequtive streams using addStream, and when no further data need to be added, the close method tells the consumer to complete its work and shut down.
This class is not just a Sink because it is also combined with
other Sink classes, like it's combined with EventSink in the
StreamSink class.
The Stream.pipe accepts a StreamConsumer and will pass the stream
to the consumer's addStream method. When that completes, it will
call close and then complete its own returned future.
- Implemented by