Attempting to reuse twisted.internet.ssl.CertificateOptions results in wrong SNI being sent

[richvdh]

@hawkowl says that it should be possible to reuse CertificateOptions for multiple TLS connections, which would be good because creating them is expensive. However, attempting to do so results in the wrong SNI being sent.

It looks like this is because CertificateOptions only uses one OpenSSL context, and ClientTLSOptions sets the set_info_callback on that one context to set the SNI. Obviously if a different ClientTLSOptions changes the callback in the meantime, then it all goes wrong.

(Note that we override ClientTLSOptions to disable cert verification, but it looks the same in the normal twisted impl.)

[richvdh]

fixed by #4674