http2: add compat trailers, adjust multi-headers

Adds Http2ServerRequest trailers & rawTrailers functionality. Also fixes
behaviour of multi-headers to conform with the spec (all values but
set-cookie and cookie should be comma delimited, cookie should be
semi-colon delimited and only set-cookie should be an array). Adds
setter for statusMessage that warns, for backwards compatibility.
End readable side of the stream on trailers or bodyless requests

Refs: https://github.com/expressjs/express/pull/3390#discussion_r136718729
PR-URL: https://github.com/nodejs/node/pull/15193


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