Move connection configuration to ConnectionConfiguration.

The setters are left behind in OkHttpClient for backwards-compatibility. But
the new best way to configure this stuff is with a ConnectionConfiguration.

That class still needs a nice new name, and perhaps tighter configuration
for string-enums (cipher suites and TLS versions).
I'm not completely convinced this is the right way to go. It _seems_ natural, but there's one big cost. If you're intentionally using multiple configurations (say, modern TLS and the fallback) you'll now need to configure all of your options for each of them: the SSL socket factory, the certificate pinner, the NPN protocols.

In theory, that flexibility is valuable. In practice, it might be annoying.

Thoughts @JakeWharton @adriancole @mescortes  ?