draft interfaces for replication admission control v2

[sumeerbhola]

This is extremely rough, and will likely be hard to understand. Meant for folks working on replication AC v2 prototype.

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[blathers-crl[bot]]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

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[sumeerbhola]

RACv2 is fully implemented. Closing this prototype.