http2: remember sent headers

Add sentHeaders, sentTrailers, and sentInfoHeaders properties on Http2Stream.

Fixes: https://github.com/nodejs/node/issues/16619

ping @mcollina

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

http2