Support SNI on TLS

[oberstet]

Intro: https://en.wikipedia.org/wiki/Server_Name_Indication

Or this (stolen from the Internet):

Server Name Indication (SNI) is an extension to the Transport Layer Security (TLS) computer networking protocol that indicates to what host name the client is attempting to connect to at the start of the handshaking process. This allows a server to present multiple certificates on the same IP address and TCP port number and hence allows multiple secure (HTTPS) websites (or any other service over TLS) to be served off of the same IP address without requiring all of those sites to use the same certificate. It is the conceptual equivalent to HTTP/1.1 virtual hosting for HTTPS.

And we should support that on universal transports, which are capable of running WAMP-WebSocket, WAMP-RawSocket, Web and soon MQTT all on the same IP/port too.

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[oberstet]

I am listing a number of resources that might be relevant in this context here:

• https://twistedmatrix.com/trac/ticket/4887
• support Deferreds from the SNI map glyph/txsni#17
• Support switching verification modes glyph/txsni#2
• Add set_verify method on Connection pyca/pyopenssl#255