http: optimize header storage and matching

This commit implements two optimizations when working with headers:

* Avoid having to explicitly "render" headers and separately store the
original casing for header names.

* Match special header names using a single regular expression instead
of testing one regular expression per header name.

PR-URL: https://github.com/nodejs/node/pull/10558


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