BUG/MEDIUM: compression: Add a flag to know the filter is still processing data

BUG/MEDIUM: compression: Add a flag to know the filter is still processing data

Since the commit acfd71b97 ("BUG/MINOR: http-comp: Preserve
HTTP_MSGF_COMPRESSIONG flag on the response"), there is no more flag to know
when the compression ends. This means it is possible to finish the
compression several time if there are trailers.

So, we reintroduce almost the same mechanism but with a dedicated flag. So
now, there is a bits field in the compression filter context.

The commit above is marked to be backported as far as 2.0. Thus this patch
must also be backported as far as 2.0.