Implement streaming Server Function calls

[ggoodman]

See the Design Doc

[Ephem]

This is very neat! I think actual gains kind of hinges on if you are using http/1.1 or 2 though, since http/2 does everything over a single connection by default and the 6 concurrent request limit doesn't apply there?

https://stackoverflow.com/questions/36835972/is-the-per-host-connection-limit-raised-with-http-2

[ggoodman]

@Ephem thanks for taking a look. That is very valuable feedback. As you can imagine, this PR introduces quite a bit of code, each line of which is adding fragility. It is hard to say whether this is being too clever (and complex) or is actually going to be a net win.

I think it will be helpful to put together a sort of degenerate case example app with lots of data fetching and run it in both http/2 and http/1.1. At the same time, such an example will be useful to demonstrate / test the Server Functions features (and meet your podcast challenge 🤣) as the framework evolves.