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.

Avoid use-after-free if key update is initiated while ppe is pending

conn_rotate_keys() frees conn->pktns->crypto.tx.ckm. If there is a pending ppe, the ppe object still holds a reference to the freed memory and will eventually try to use it in ngtcp2_ppe_final().

Fix use of uninitialized data in tests