Pass struct sockaddr to ngtcp2 so that it knows how to deal with
address change in more detailed manner (e.g., just compare IP address
without port).

Remove conn and user_data from encrypt/decrypt/hp_mask callbacks so
that they can be used without conn.

write_single_frame_pkt() returns a packet length greater than the number of bytes it writes to the buffer. The returned length reflects a fake AEAD overhead, while this extra overhead is never written. As a result, ngtcp2_verify_stateless_reset_token() uses initialized data when reading the end of the packet. This causes noise when using a memory sanitizer.

Require token if the first packet is lost or reordered in order to
buffer the data coming from validated client only.

The interpretation of Additional ACK Blocks has to be changed in a
later commits.

stream_id is still in uint32_t.