gRPC: improved keepalive handling.
gRPC: improved keepalive handling.

The code is now able to parse additional control frames after
the response is received, and can send control frames as well.
This fixes keepalive problems as observed with grpc-c, which can
send window update and ping frames after the response, see
http://mailman.nginx.org/pipermail/nginx/2018-August/056620.html.

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx