Add support for dynamic HTML

[devknoll]

Implements renderToString in terms of a new renderToStream. The former is used for legacy documents that generate the body HTML as part of getInitialProps. The latter will be used directly in #27794 when streaming dynamic HTML.

Since we're exposing an actual streaming response for dynamic HTML (instead of buffering with resultFromChunks), we use multiplexResult to buffer and multiplex the underlying result to multiple subscribers.

[huozhi]

is this comment still needed?

[huozhi]

not sure if it need a try-catch here like line

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
buildDuration	12.8s	12.8s	⚠️ +59ms
buildDurationCached	3s	2.9s	-52ms
nodeModulesSize	49.1 MB	49.1 MB	⚠️ +7.96 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.308	2.324	⚠️ +0.02
/ avg req/sec	1083.38	1075.72	⚠️ -7.66
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.286	1.318	⚠️ +0.03
/error-in-render avg req/sec	1943.47	1897.31	⚠️ -46.16

Client Bundles (main, webpack, commons)

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
745.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	23.1 kB	23.1 kB	✓
webpack-HASH.js gzip	1.5 kB	1.5 kB	✓
Overall change	67 kB	67 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
polyfills-HASH.js gzip	31.1 kB	31.1 kB	✓
Overall change	31.1 kB	31.1 kB	✓

Client Pages

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
_app-HASH.js gzip	980 B	980 B	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	329 B	329 B	✓
dynamic-HASH.js gzip	2.52 kB	2.52 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	904 B	904 B	✓
image-HASH.js gzip	4.13 kB	4.13 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	319 B	319 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	12.8 kB	12.8 kB	✓

Client Build Manifests

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
_buildManifest.js gzip	491 B	491 B	✓
Overall change	491 B	491 B	✓

Rendered Page Sizes

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
index.html gzip	533 B	533 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	525 B	525 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Webpack 4 Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
buildDuration	10.6s	10.9s	⚠️ +278ms
buildDurationCached	4.1s	4.1s	⚠️ +3ms
nodeModulesSize	49.1 MB	49.1 MB	⚠️ +7.96 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.352	2.342	-0.01
/ avg req/sec	1063.02	1067.35	+4.33
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.295	1.284	-0.01
/error-in-render avg req/sec	1931.16	1946.32	+15.16

Client Bundles (main, webpack, commons)

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
17.HASH.js gzip	185 B	185 B	✓
677f882d2ed8..HASH.js gzip	14 kB	14 kB	✓
framework.HASH.js gzip	41.9 kB	41.9 kB	✓
main-HASH.js gzip	10.6 kB	10.6 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	67.9 kB	67.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
polyfills-HASH.js gzip	31.3 kB	31.3 kB	✓
Overall change	31.3 kB	31.3 kB	✓

Client Pages

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
_app-HASH.js gzip	965 B	965 B	✓
_error-HASH.js gzip	3.71 kB	3.71 kB	✓
amp-HASH.js gzip	552 B	552 B	✓
css-HASH.js gzip	333 B	333 B	✓
dynamic-HASH.js gzip	2.71 kB	2.71 kB	✓
head-HASH.js gzip	2.97 kB	2.97 kB	✓
hooks-HASH.js gzip	911 B	911 B	✓
index-HASH.js gzip	231 B	231 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	298 B	298 B	✓
script-HASH.js gzip	2.95 kB	2.95 kB	✓
withRouter-HASH.js gzip	294 B	294 B	✓
e025d2764813..52f.css gzip	125 B	125 B	✓
Overall change	17.7 kB	17.7 kB	✓

Client Build Manifests

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
_buildManifest.js gzip	498 B	498 B	✓
Overall change	498 B	498 B	✓

Rendered Page Sizes

	vercel/next.js canary	azukaru/next.js x-render-to-stream	Change
index.html gzip	578 B	578 B	✓
link.html gzip	591 B	591 B	✓
withRouter.html gzip	570 B	570 B	✓
Overall change	1.74 kB	1.74 kB	✓

Commit: d17b6b5