OkHttp Client stream and transport are tightly coupled

The interaction between the client stream and transport for OkHttp stream starting is tightly coupled. As brought up in #1294, we should take a look at how these two classes interact and make some clear boundaries (or possibly merge them).

Start by reading this issue alongside #1294, then trace how the OkHttp client stream and transport interact during stream startup. Done means the responsibilities have clear boundaries, or the classes are intentionally merged, with the resulting design agreed and implemented.

Written by the indexing model from the issue text.