SSL object refactoring using SSL_CONNECTION object

Make the SSL object polymorphic based on whether this is
a traditional SSL connection, QUIC connection, or later
to be implemented a QUIC stream.

It requires adding if after every SSL_CONNECTION_FROM_SSL() call
which itself has to be added to almost every public SSL_ API call.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18612)