MEDIUM: htx: Refactor htx_xfer_blks() to not rely on hdrs_bytes field

MEDIUM: htx: Refactor htx_xfer_blks() to not rely on hdrs_bytes field

It is the only function using the hdrs_bytes start-line field. Thus the
function has been refactored to no longer rely on it. To do so, we first
copy HTX blocks to the destination message, without removing them from the
source message. If the copy is interrupted on headers or trailers, we roll
back. Otherwise, data are drained from the source buffer.

Most of time, the copy will succeeds. So the roll back is only performed in
the worst but very rare case.