Mandate flow control in bytes

[bboston7]

Resolves #3781 and #4374.

This change removes the ENABLE_FLOW_CONTROL_BYTES config option and always uses flow control in bytes.

Checklist

• [x] Reviewed the contributing document
• [x] Rebased on top of master (no merge commits)
• [x] Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• [x] Compiles
• [x] Ran all tests
• [ ] If change impacts performance, include supporting evidence per the performance document

[bboston7]

I fully removed the option to disable flow control in bytes and simplified the flow control code. I also bumped the minimum overlay version. This is now ready for another round of review!