PING frame is only allowed in 1RTT encryption level.  CRYPTO frame is
not allowed in 0RTT encryption level.

Client should receive and process Short packet if it has
server_application_traffic secret.

Remove conn->early_rtb.  early data should be added to ngtcp2_rtb.

TODO There are functions which does not check return value of
ngtcp2_ksl_{add,remove}.

examples: Support writing new keylog format

Since draft -13, the exporter secret has become useless for decryption.
While the client and server programs log appropriate secrets using the
"-s" option, let's make integration with Wireshark easier by including
the appropriate labels and identifier (client random).

The corresponding Wireshark patch that uses these secrets:
https://code.wireshark.org/review/29677
(tested with ngtcp2 master (draft -13) and draft-14)