Use `Writable` instead of `Observable` and remove the `zen-observable` dependencies. I initially opted to use `Observable` for simplicity and fast iteration, but we should really just use `Writable` directly (or some other stream in the future).
React's streaming SSR has some [specific requirements](https://github.com/reactwg/react-18/discussions/66#discussioncomment-944266) on the stream API. Rather than trying to also squeeze a `Readable` in here, which might be more standard for node apps, I've just followed React's lead. By limiting ourselves to just `Writable`, it ought to be easier to adopt a different stream type in the future if desired.
The React `pipeToNodeWritable` API requires us to pass a stream immediately, but we don't actually have a `ServerResponse` to give it until `RenderResult.pipe(...)` is called later. For that reason, we pass React a `Writable` that we will simply forward to `res` later. This mechanism of deferring is `NodeWritablePiper`, which is just a function that can be called with `ServerResponse` (or another `Writable`, as we now do to render to string for static results) to have content written to it. `NodeWritablePiper` takes a `next` argument so that we can chain both synchronous and asynchronous pipers together.
Also does some clean up and adds another streaming test for backpressure.
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