MEDIUM: htx: Improve detection of fragmented/unordered HTX messages

MEDIUM: htx: Improve detection of fragmented/unordered HTX messages

First, an HTX flags was added to know when blocks are unordered. It may
happen when a header is added while part of the payload was already received
or when the start-line is replaced by an new one. In these cases, the blocks
indexes are in the right order but not the blocks payload. Knowing a message
is unordered can be useful to trigger a defragmentation, mainly to be able
to append data properly for instance.

Then, detection of fragmented messages was improved, especially when a
header or a start-line is replaced by a new one.

Finally, when data are added in a message and cannot be appended into the
previous DATA block because the message is not aligned, a defragmentation is
performed to realign the message and append data.