MEDIUM: cache: Don't rely on a chunk to store messages payload

MEDIUM: cache: Don't rely on a chunk to store messages payload

When the response payload is stored in the cache, we can avoid to use a
trash chunk as temporary space area before copying everything in the cache
in one call. Instead we can directly write each HTX block in the cache, one
by one. It should not be an issue because, most of time, there is only one
DATA block.

This commit depends on "BUG/MEDIUM: shctx: Use the next block when data
exactly filled a block".