Not sure why the retry logic was removed since 0-RTT works even in
presence of the special alerts handling (alertEndOfEarlyData is
processed before the warning alert). To reduce divergence from upstream
code (which adds a restriction on the number of consecutive warnings),
restore the original retry logic.
Do not do anything fancy here, later drafts will remove the special
alert handling since it becomes a special handshake message.
Fixes: ("crypto/tls: implement TLS 1.3 server 0-RTT")