nghttpx: Submit RST_STREAM if HEADERS appears in the middle of a stream
Just ignore HEADERS with non-idle stream ID and not found in stream map
Issue connection error if client receives HEADERS with idle stream ID

If stream ID is not idle, it might be valid HEADERS.  If stream ID is
idle, it is invalid regardless stream ID is even or odd, since client
is not expected to recieve request from server.  nghttp2 library
historically allows this, but now we forbids this.