HTTP/2: rewritten handling of request body.

HTTP/2: rewritten handling of request body.

There are two improvements:

  1. Support for request body filters;

  2. Receiving of request body is started only after
     the ngx_http_read_client_request_body() call.

The last one fixes the problem when the client_max_body_size value might not be
respected from the right location if the location was changed either during the
process of receiving body or after the whole body had been received.