server: Override priority with u and i query parameters
server: Override priority with u and i query parameters

examples/server accepts 2 new query parameters.

- u query parameter takes urgency value [0, 7], inclusive (e.g., u=0).

- i query parameter takes 0 or 1, and they present non-incremental and
  incremental respectively (e.g., i=1).  Note that this is not
  sh-boolean.

The server overrides stream priority with these parameters.

ngtcp2/ngtcp2,ngtcp2/ngtcp2,ngtcp2/ngtcp2