Fix condition checking for Initial packet

The code previously determined whether a packet was an Initial packet by looking at its type value, without first checking it was not a Version Negotiation packet. In the case of a Version Negotiation packet, reading the type value returned the random value in the Unused field. This occasionally caused Version Negotiation packets to be misinterpreted as Initial packets. When this happened, ngtcp2_conn_read_pkt() returned NGTCP2_ERR_DRAINING instead of NGTCP2_ERR_RECV_VERSION_NEGOTIATION.

Return dest->len from ngtcp2_pkt_decode_padding_frame

Add comment to clarify len in pkt_decode_version_cid

Refs: https://github.com/ngtcp2/ngtcp2/issues/191

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

Add generated certificates to .gitignore

Introduce put_uint16_varint function

Remove unnecessary callback comments

Remove unused chrono include from debug.h

Replace goto in create_ssl_ctx with exit

This commit adds a static function named put_uint16_varint to reduce
some code duplication when putting some transport parameters.

chrono is used by generate_token and verify_token and was previously
indirectly via debug.h.

This commit updates the server's create_ssl_ctx use exit(EXIT_FAILURE)
instead of using a goto label.

The motivation of this is to be consistent with create_ssl_ctx in
client.cc.