BUG/MAJOR: filters/htx: Add a flag to state the payload is altered by a filter

BUG/MAJOR: filters/htx: Add a flag to state the payload is altered by a filter

When a filter is registered on the data, it means it may change the payload
length by rewritting data. It means consumers of the message cannot trust the
expected length of payload as announced by the producer. The commit 8bd835b2d2
("MEDIUM: filters/htx: Don't rely on HTX extra field if payload is filtered")
was pushed to solve this issue. When the HTTP payload of a message is filtered,
the extra field is set to 0 to be sure it will never be used by error by any
consumer. However, it is not enough.

Indeed, the filters must be called before fowarding some data. They cannot be
by-passed. But if a consumer is unable to flush the HTX message, some outgoing
data can remain blocked in the channel's buffer. If some new data are then
pushed because there is some room in the channel's buffe, the producer will set
the HTX extra field. At this stage, if the consumer is unblocked and can send
again data, it is possible to call it to forward outgoing data blocked in the
channel's buffer before waking the stream up to filter new input data. It is the
purpose of the data fast-forwarding. In this case, the HTX extra field will be
seen by the consumer. It is unexpected and leads to undefined behavior.

One consequence of this bug is to perform a wrong chunking on compressed
messages, leading to processing errors at the end of the message, reported as
"ID--" in logs.

To fix the bug, a HTX flag is added to state the payload of the current HTX
message is altered. When this flag is set (HTX_FL_ALTERED_PAYLOAD), the HTX
extra field must not be trusted. And to keep things simple, when this flag is
set, the HTX extra field is automatically set to 0 when the HTX message is
loaded, in htxbuf() function.

It is probably the less intrusive way to fix the bug for now. But this part must
be reviewed to save meta-info of the HTX message outside of the message itself.

This commit should solve the issue #2741. It must be backported as far as 2.9.

(cherry picked from commit 52a3d807fc332b57b62f5e30aa6f697636a22695)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>