It now defaults to 2 and includes CID exchanged during handshake.

The changed semantics will be implemented in the later commits.

Make ngtcp2_pkt_write_stateless_reset use const for input

These two input buffers are not modified as part of the function
and can/should thus be `const`.

* `ngtcp2_crypto_generate_stateless_reset_token` - Used to generate
  a stateless reset token as an HKDF extraction using the CID and
  a token secret as input.

Remove unused color-related code from examples

Fix typo

Fix example server's write callback never being called

Handler::wev_ is initialized with a file descriptor set to 0. ev_io_set() is never called for this watcher, so its file descriptor remains 0 and calls to ev_io_start() don't have the desired effect. As a result writecb() is never called.

There may still be an issue due to all endpoints sharing the same watcher, but this is mostly orthogonal to the problem fixed here.