Introduce ConnectionConfiguration.

Expand the scope of TlsConfiguration to all connections, http: and https:.

Closes https://github.com/square/okhttp/issues/815
Closes https://github.com/square/okhttp/issues/1058
Reworded slightly. (It's the proxy servers are being attempted; it might have made more sense if the noun was "challenges", like "Prepares the challenges to attempt".)