asyncio: Add "call_connection_made" arg to SSLProtocol.__init__

Issue #23749: With this change it's possible to implement starttls
as a separate package on PyPI, or even by copying/pasting a small
snipped of code in your project.

It's expected that we'll figure out the API design for starttls
during 3.6, so that we can add it in 3.7.