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implement the new handshake

When resuming a session using session tickets, no certificate chain is
presented, and the callbacks needed to verify the peer identity would
not be called.

use a prefix when signing the public key

use ChaCha if one of the peers doesn't have AES hardware support

Co-Authored-By: marten-seemann <martenseemann@gmail.com>

improve peer verification

add an example (mainly for development)

use TLS 1.3

TLS 1.3 support was recently made opt-in in Go 1.12, so we need to
explicitly enable it.

TLS 1.3 will require Go >= 1.12, so we don't need any fixes that were
specific to Go 1.10 any more.

remove unneeded marshaling / unmarshaling when generating cert chain

avoid using interface{} when generating certificates

add support for ECDSA keys

close the underlying connection when the context is canceled