Implement Connection.startssl(ssl_context) coroutine

[asvetlov]

Prerequirement for #2019

The change makes sense itself -- it simplifies upgrade SSL proxy connection and makes code cleaner.

[fafhrd91]

do you expect anyone would create proxy with aiohttp?

[asvetlov]

No, I'm thinking about aiohttp internals refactoring only.

Now TCPConnector for SSL upgrade does the following:

1. Create a Connection for sending CONNECT request without registering it in connection pool.
2. Clean up Connection._transport (BTW not used anymore attribute) and Connection._protocol
3. Negotiate TLS connection by calling loop.create_connection with duplicated socket.
4. Close transport
5. Return a new protocol given from TLS

It works but looks cumbersome.
I recall request for SSL upgrade for UNIX sockets. I don't want to copy all this code to UNIX Connector.

Let me experiment with implementation and make a PR.
If I've found the idea is dead -- I reject it.
But now moving ssl upgrade from connector layer to ClientSession._request makes sense in my mind.

[fafhrd91]

initially I thought about different use case. now it makes more sense.

[asvetlov]

Let's close the issue, it should be created on new basis

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new