MEDIUM: htx: Be sure size of headers never exceed regular buffer on update

MEDIUM: htx: Be sure size of headers never exceed regular buffer on update

When an HTX header or an HTX start-line is added or updated, a test is now
performed to be sure the whole headers size, including the start-line never
exceeds the regular buffer size. It is mandatory because it is now possible
to use larger buffers. However, it remains impossible to send HEADERS frame
in H2 and QUIC larger than a regular buffer. So we must be sure the HTTP
analysis will never produce too big headers because they will be rejected
later, at the forwarding stage. The purpose of large buffers is to be able
to store large payload, larger than regular buffers. Headers must remain
quite small.

This commit depends on the following one:

  MINOR: htx: Add a field to save the headers data size

Both should be backported to 3.4 to avoid any trouble with large buffers. It
is not strictly speaking a bug, but it will avoid hazardous behavior
depending on the payload size.