http2: use native pipe instead of synchronous I/O

This resolves the issue of using synchronous I/O for
`respondWithFile()` and `respondWithFD()`, and enables
scenarios in which the underlying file does not need
to be a regular file.

PR-URL: https://github.com/nodejs/node/pull/18936


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>