[wip] Cut various support for ancient servers

[gnprice]

This is a rebase of the draft branch I wrote a little over a year ago for this purpose. After writing it I realized that much of it should block on refusing to connect to ancient servers. We've now done that, in #5633.

This still needs some polishing-up, and the last few commits are to rely on server 2.1 which should wait for a sequel to #5633. But other than those last few, I think it's pretty close to mergeable.

[chrisbobbe]

Thanks! I confirmed that tools/test --diff @~ passes on all the commits, and the ones not marked "wip" look good to me.

For the TODO(server-2.1)s, I think it would be reasonable add a commit that bumps kMinAllowedServerVersion from 2.0 to 2.1.

Then another search for TODO(server-2.0) and TODO(server-2.1) will find a few that were added within the last year, so after you drafted this; I expect they'd be easily addressed.