BUG/MINOR: h3/qpack: deal with too many headers

BUG/MINOR: h3/qpack: deal with too many headers

ensures that we never insert too many entries in a headers input list.
On the decoding side, a new error QPACK_ERR_TOO_LARGE is reported in
this case.

This prevents crash if headers number on a H3 request or response is
superior to tune.http.maxhdr config value. Previously, a crash would
occur in QPACK decoding function.

Note that the process still crashes later with ABORT_NOW() because error
reporting on frame parsing is not implemented for now. It should be
treated with a RESET_STREAM frame in most cases.

This can be backported up to 2.6.