Once we have legacy/trac#19301 complete, we can update clients to generate circuits using ed25519 identities. We'll have to actually store the ed25519 identity in the extend_cell_t when we make in . We'll have to encode it as part of extend_cell_format. We'll have to decide whether to use it: it's only okay to send the ed25519 ID when both servers support the new link handshake. We can enable this with a tristate, for testing, and to make sure that this turns on for a big pile of clients at once.