Fix SSL_get_servername() and SNI behaviour

Fix SSL_get_servername() and SNI behaviour

The SNI behaviour for TLSv1.3 and the behaviour of SSL_get_servername()
was not quite right, and not entirely consistent with the RFC.

The TLSv1.3 RFC explicitly says that SNI is negotiated on each handshake
and the server is not required to associate it with the session. This was
not quite reflected in the code so we fix that.

Additionally there were some additional checks around early_data checking
that the SNI between the original session and this session were
consistent. In fact the RFC does not require any such checks, so they are
removed.

Finally the behaviour of SSL_get_servername() was not quite right. The
behaviour was not consistent between resumption and normal handshakes,
and also not quite consistent with historical behaviour. We clarify the
behaviour in various scenarios and also attempt to make it match historical
behaviour as closely as possible.

Fixes #8822

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/10018)