Avoid calling onServerNameIndication twice with HRR

[ocheron]

When TLS13 is negotiated, the callback onServerNameIndication in called too early in the server code flow. In case of HelloRetryRequest there is an unnecessary call for the first Client Hello. It is needed only for the second Client Hello.

To avoid this, the PR moves the call to splitted implementations TLS12/TLS13:

• Knowning what credentials are available is required for cipher selection < TLS13
• With TLS13, onServerNameIndication can be called after HRR handling

Test case "SNI" is also extended to verify that onServerNameIndication is called once, and that getClientSNI returns a correct result on client side.

[kazu-yamamoto]

Looks excellent to me!

[kazu-yamamoto]

Merged.

[ocheron]

Yes I saw that when trying Encrypted SNI.
We could even go one step further and remove the call completely for a PSK handshake.
But some people might use the callback for other goal than providing credentials.