parse certificateverify message
default for assemble_list now to produce a buffer (with list length encoded); if none_if_empty provided, return a 0-length buffer
asemble certificate_request
provide assemble_certificate_request and assemble_certificate_request_1_2
style
move peer_rsa_key, verify_digitally_signed, and validate_chain to handshake_common
accept authenticator in server config
server handshake states dealing with client authentication
provide client authentication functionality:
 - if authenticator is configured, ask for a client certificate (send CertificateRequest)
 - if certificate is provided by client, validate the chain (using the authenticator)
 - verify the signature (using the client certificate key) of the CertificateVerify message
 - store client certificate in session.peer_certificate (and trust anchor in session.trust_anchor)

the application still needs to decide whether the session.peer_certificate is good for it (it might be empty!!!)
echo client sends a client certificate if asked, echo_server requests a client certificate