Implement streaming operator

[senderista]

The design is described in uwescience/myria#858. RACO does the work of translating a stream(relationVar) statement into a logical Stream operator, and expanding that into a physical operator chain CollectProducer->CollectConsumer->StreamingSink, while MyriaX expands StreamingSink into a TupleSink with PipeSink instances that register their InputStreams under the running query, so we can construct an HTTP streaming response entity with the sequenced output of all stream() statements.

[coveralls]

Coverage decreased (-0.2%) to 91.601% when pulling adba8c8 on streaming_query_results into cf26abe on master.

[coveralls]

Coverage decreased (-0.1%) to 91.565% when pulling ad97764 on streaming_query_results into c6fdede on master.

[coveralls]

Coverage decreased (-0.1%) to 91.565% when pulling ad97764 on streaming_query_results into c6fdede on master.