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

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

[SirTyson]

LGTM, one comment on changing a flow control bytes check to make deprecation easier.

[SirTyson]

LGTM, thanks for the changes!

[marta-lokhova]

I think you can go further with this change and remove support for disabling flow control in bytes completely:

• Minimum overlay version is far beyond FIRST_VERSION_SUPPORTING_FLOW_CONTROL_IN_BYTES at this point, so most (if not all) nodes on the network will have the feature enabled
• I doubt there are any nodes outside of our infra that are manually tuning this config. Looking at a snapshot of peers on one of the watchers, I don't see anyone not using the feature.
• I think we can remove any conditional enabling of flow control in bytes, and just implicitly set it to true. You might be able to get rid of the annoying FlowControl::start call at the end of authentication as well, and basically start flow control automatically when we create Peer.
• We also don't need to set AUTH_MSG_FLAG_FLOW_CONTROL_BYTES_REQUESTED anymore. I'd suggest we set auth flags back to 0, so we can use it for future feature rollout. AUTH_MSG_FLAG_FLOW_CONTROL_BYTES_REQUESTED can be deprecated.
• We still need to bump overlay version. While we're at it: the network recently upgraded to v21, so we can actually bump minimum overlay version as well to the one that supports v21 (meaning that we can drop all v20 clients now)
• Dropping support for this flag means we can drop all the tests dealing with mixed modes. Yay to code removal!

[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!