BUG/MEDIUM: compression: fix incompatibility between compression and large-buffers

BUG/MEDIUM: compression: fix incompatibility between compression and large-buffers

The SLZ compression being stateless, it doesn't keep data between calls
and must be called with the largest blocks at once to be efficient. When
it was added, muxes were not there yet and chunks were parsed inside the
HTTP forwarding analyzer, which would call comp->add_data() for each chunk
of data. In case of small chunks, that would just not compress, so a local
buffer was added and data were temporarily stored there and effectively
compressed when calling ->flush().

But with the introduction of large buffers, it became a problem because
the locally allocated buffer can be too small if the data to be compressed
comes from a large one. One solution to this is to also pre-allocate a
large buffer, but since there needs to be one per thread and that some
users might use extremely large buffers on large systems, that could be
extremely expensive in terms of memory.

A better solution is to just simplify all of this as this complexity is
no longer needed at the era of muxes which extract chunks and make a
large HTX data block with them. That's what this patch does, and the
functions become as simple as their zlib counterparts, no longer require
a local per-thread buffer, and will be even faster thanks to the removal
of a long-unneeded extra copy.

This fix will need to be backported to 3.4 where large buffers were
introduced, maybe after a period of observation. An alternate fix in
the mean time could be to reject configs having both compression and
large buffers.