Whenever we rotate our TLS context, we change our Ed25519
Signing->Link certificate.  But if we've already started a TLS
connection, then we've already sent the old X509 link certificate,
so the new Ed25519 Signing->Link certificate won't match it.

To fix this, we now store a copy of the Signing->Link certificate
when we initialize the handshake state, and send that certificate
as part of our CERTS cell.

Fixes one case of bug22460; bugfix on 0.3.0.1-alpha.