The r->request_body_no_buffering flag was introduced.  It instructs
client request body reading code to avoid reading the whole body, and
to call post_handler early instead.  The caller should use the
ngx_http_read_unbuffered_request_body() function to read remaining
parts of the body.

Upstream module is now able to use this mode, if configured with
the proxy_request_buffering directive.

Now we log a "zero size buf in chain writer" alert if we encounter a zero
sized buffer in ngx_chain_writer(), and skip the buffer.