Send dummy ChangeCipherSpec messages from the TLSv1.3 server

If the client has requested middle box compatibility mode by sending
a non-empty legacy_session_id, the server must send a dummy CCS right
after its first handshake message. This means right after ServerHello
or HelloRetryRequest.

Two important improvements over the backed-out diffr: make sure that
First: client and server can send their dummy CCS at the correct moment
(right before the next flight or right after the current flight).
Second: as jsing noted, we also need to deal with the corner case that
tls13_send_dummy_ccs() can return TLS13_IO_WANT_POLLOUT.

with/ok jsing

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