* Drop DC test data for draft23

* DC: Change public key type

There was a last minute change to spec that changes the public key type
from a byte string of length at most 2^16 to a byte string of length at
most 2^24.

Drops support for delegated credentials with TLS 1.2 and adds the
protocol version and signature algorithm to the credential structure.

What's left are the minimal API changes required to use the delegated
credential extension in the TLS handshake.

This complies with the standard, currently an Internet draft:
https://tlswg.github.io/tls-subcerts/draft-ietf-tls-subcerts.html

It also adds a minimal interface for generating new delegated
credentials.