Request body: unbuffered reading.

Request body: unbuffered reading.

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.