A SubFlow (or SubSource) is not a Graph, it is an unfinished builder
that accepts transformations. This allows us to capture the substreams’
transformations before materializing the flow, which will be very
helpful in fully fusing all operators.
Another change is that groupBy now requires a maxSubstreams parameter in
order to bound its resource usage. In exchange the matching merge can be
unbounded. This trades silent deadlock for explicit stream failure.
This commit also changes all uses of Predef.identity to use `conforms`
and removes the HTTP impl.util.identityFunc.
main work by @drewhk with contributions from @2m and @rkuhn
This work uncovered many well-hidden bugs in existing Stages, in
particular StatefulStage. These were hidden by the behavior of
OneBoundedInterpreter that normally behaves more orderly than it
guarantees in general, especially with respect to the timeliness of
delivery of upstream termination signals; the bugs were then that
internal state was not flushed when onComplete arrived “too early”.
