* Write BUILD_ID when building.
It's a random id (uuid.v4())
* Add buildId to the core JS files.
* Add immutable cache-control header.
Only if the buildId is matched.
* Set '-' as the dev buildId always.
* Add buildId handling for JSON pages.
* Add AOT gzip content-encoding support.
Currently we only do this for
main.js and commons.js only.
* Remove unwanted await.
* Use Promise.all to gzip assets in parallel.
* compile _document using webpack
* don't emit the bundle file of _document.js
* exclude _document.js from minChunks of CommonsChunkPlugin
* handle creation/removal of pages/_document.js
* improve path handlings