API: Consider removing the ability of streams to build stream graphs

[branimirangelov]

With the new notation for modules and the ability to have multiple HTTP methods in a module, stream graph building can be delegate to modules.

[bojidar-bg]

With the merging of streams and calls/modules into executions in 884a80a, I think such a limitation would not make much sense.
However, as creating a stream is currently not an idempotent operation, doing it in a stream could potentially lead to a phantom stream existing after a well-timed crash.

Removing from the 0.6 milestone, as it is not blocking for release. 😃