HTTP/3: send Stream Cancellation instruction.

HTTP/3: send Stream Cancellation instruction.

As per quic-qpack-21:

   When a stream is reset or reading is abandoned, the decoder emits a
   Stream Cancellation instruction.

Previously the instruction was not sent.  Now it's sent when closing QUIC
stream connection if dynamic table capacity is non-zero and eof was not
received from client.  The latter condition means that a trailers section
may still be on its way from client and the stream needs to be cancelled.