ServiceWorker lifetime and respondWith() with a user-constructed ReadableStream

Branched from https://github.com/yutakahirano/fetch-with-streams/issues/63#issuecomment-207453714

respondWith() with a user-constructed ReadableStream

When a user-constructed ReadableStream is being consumed by respondWith(), respondWith() doesn't know what's happening inside the ReadableStream. The ReadableStream interface of the instance facing respondWith() doesn't allow either scraping out its contents or knowing ReadableStreamDefaultController.close() is invoked (note that a ReadableStream doesn't always have ReadableStreamDefaultController behind it).

Until all the chunks are read out from the ReadableStream, it's holding some JS objects representing the underlying source and chunks in its queue. So, we want to keep the ServiceWorker alive until they're drained.

Also, note that only respondWith() can observe the ReadableStream's state when respondWith() is consuming it. It could work but is not good that we tell the instance to wait for to SW like evt.waitUntilClosed(aStream) as Ben suggested at https://github.com/yutakahirano/fetch-with-streams/issues/63#issuecomment-207487623.

evt.respondWith(response, { waitForBody: true }); (https://github.com/yutakahirano/fetch-with-streams/issues/63#issuecomment-207489289) by Ben looks better.

There we could have a timeout to abort receiving chunks even when waitForBody is specified so that badly behaving stream doesn't prolong SW lifetime forever.

respondWith() with a ReadableStream constructed by Fetch API

I agree with Ben's point at https://github.com/yutakahirano/fetch-with-streams/issues/63#issuecomment-207483430. This is also related to the discussion how we realize efficient pipeTo() algorithm. I.e. for certain operations, streams would behave like a simple identifier of data source / destination like file descriptor.

I'll try to standardize all the interfaces for these issues by resuming the pipeTo() discussion threads.

No repository files or tests are named. Start by reviewing the linked fetch-with-streams issue 63 and the discussion of respondWith(), ReadableStream, waitUntilClosed(), waitForBody, and pipeTo(). Done requires a settled interface and ServiceWorker lifetime behavior suitable for standardization.