use chacha20poly1305 instead of AES and include cert in key derivation

Unfortunately QUIC uses non-standard tag sizes with both AES-GCM and
Poly1305. Adopting AES-GCM seems much harder, so I changed it to
Chacha20Poly1305 and only made some slight changes to an existing algo.
This should probably be double-checked at some point.

crypto/proof_rsa.go

@@ -70,3 +70,8 @@ func (kd *KeyData) GetCERTdata() []byte {
 	gz.Close()
 	return b.Bytes()
 }
+
+// GetCertUncompressed gets the certificate in DER
+func (kd *KeyData) GetCertUncompressed() []byte {
+	return kd.cert.Raw
+}