BUG/MEDIUM: flt-http-comp: Block HTX_FL_EOM flag till the end of compression

BUG/MEDIUM: flt-http-comp: Block HTX_FL_EOM flag till the end of compression

Because the compression filter is generating a new payload and especially
because data may be blocked in its internal context, the EOM flag must be
removed from the HTX message and only set again at the end of the
compression.

It is important to be sure the end of message is not detected too early.
Indeed, it may happen if some data are blocked in the filter context but all
data in the message must be fully forwarded. In that case, we have a message
with no input at all and the EOM flag. It is the condition to consider the
end of the message was reached. This could lead to truncate the message.

To fix the issue, the compression filter now remove systematically the EOM
flag from the HTX message. But it save the information in its context,
thanks to the COMP_STATE_EOM_SEEN flag. This way, at the end of the
compression, it can set it again.

This patch could be backported to supported version. Not sure the bug was
ever been observed. But it is theorically possible.