http2: use a reference counter for headers

http2: use a reference counter for headers

Ticket: 6892

As HTTP hpack header compression allows one single byte to
express a previously seen arbitrary-size header block (name+value)
we should avoid to copy the vectors data, but just point
to the same data, while reamining memory safe, even in the case
of later headers eviction from the dybnamic table.

Rust std solution is Rc, and the use of clone, so long as the
data is accessed by only one thread.