MEDIUM: HTTP compression (zlib library support)

This commit introduces HTTP compression using the zlib library.

http_response_forward_body has been modified to call the compression

This feature includes 3 algorithms: identity, gzip and deflate:

  * identity: this is mostly for debugging, and it was useful for
  developping the compression feature. With Content-Length in input, it
  is making each chunk with the data available in the current buffer.
  With chunks in input, it is rechunking, the output chunks will be
  bigger or smaller depending of the size of the input chunk and the
  size of the buffer. Identity does not apply any change on data.

  * gzip: same as identity, but applying a gzip compression. The data
  are deflated using the Z_NO_FLUSH flag in zlib. When there is no more
  data in the input buffer, it flushes the data in the output buffer
  (Z_SYNC_FLUSH). At the end of data, when it receives the last chunk in
  input, or when there is no more data to read, it writes the end of
  data with Z_FINISH and the ending chunk.

  * deflate: same as gzip, but with deflate algorithm and zlib format.
  Note that this algorithm has ambiguous support on many browsers and
  no support at all from recent ones. It is strongly recommended not
  to use it for anything else than experimentation.

You can't choose the compression ratio at the moment, it will be set to
Z_BEST_SPEED (1), as tests have shown very little benefit in terms of
compression ration when going above for HTML contents, at the cost of
a massive CPU impact.

Compression will be activated depending of the Accept-Encoding request
header. With identity, it does not take care of that header.

To build HAProxy with zlib support, use USE_ZLIB=1 in the make

This work was initially started by David Du Colombier at Exceliance.
11 files changed