Router: allow methods to return an async-iterator or a readable stream

[bajtos]

Right now, Controller methods are returning a single response object that's later written to the HTTP response body.

Let's add support for two more advanced response modes:

• A method returning an async iterator instead of in-memory array instance - see e.g. KeyValueModel.iterateKeys

• A method returning a readable stream. The stream can be either "regular" text/binary stream, but also a stream in object mode, where the framework is expected to serialize the objects to JSON/XML/etc.

[bajtos]

In #97 (comment), @raymondfeng suggested we may want to leverage https://github.com/dominictarr/JSONStream in the implementation.

[raymondfeng]

+1. A lot of DB drivers allow row by row access to a large result set.

From the programming model perspective, we should explore the possibility to support:

1. Streams
2. Async Iterators (push)
3. Generators (pull)
4. Pub/sub

[bajtos]

I am closing this issue because we are not going to have time to work on this in the next 6-9 months. However, we are happy to accept a pull request contributing this feature.