TLS 1.3 has no renegotiation and provides transparent rekeys but this
was not used by the server. Instead rekey-method=ssl was silently
downgraded to new-tunnel, causing a full tunnel/TUN-device rebuild (and
a brief data-path interruption) on every rekey whenever a client
negotiated TLS 1.3.
This commit simplifies that handling by taking advantage of TLS 1.3's own
rekey mechanism instead: the server now performs the "ssl" rekey on
TLS 1.3 sessions via the standard TLS 1.3 KeyUpdate message.
TLS <= 1.2 rekey behavior (client-driven rehandshake, gated on RFC
5746 safe renegotiation) is unchanged.
Resolves: #745
Signed-off-by: Nikos Mavrogiannopoulos <n.mavrogiannopoulos@gmail.com>
