Basic cc integration

1. Basic version of CC integrated with upstream.
2. Interface simplification for sending msgs.
3. Many bug fixes.

Passes tst, poc with 350/512, 512/1024 configurations.
Doesn't pass my internal cc-poc, perhaps due to integration with the 3.9 upstream. I'm investigating, but it should not interfere with anyone's work.

No changes to any existing APIs - the default mode is the shared-nothing, backwards-compatible mode, unless explicitly passed mr.flags |= UD_SHARED.