Initial cherrypicks for 0.12.1 - Part 1

[kbendick]

Some of the initial PRs that we're cherry-picking for 0.12.1 branch.

This is only the PRs that cherry-pick cleanly.

[kbendick]

I skipped over the following 2 PRs as they don't merge cleanly:

• Core: Validate concurrently added delete files in RowDelta #3195
• Core: Validate concurrently added delete files in OverwriteFiles #3199

After this, some PRs don't merge cleanly only because of the restructure. So I will edit those to be in line with the older structure (and then there's no difference).

I have a small script that can rebuild this at any time so if we want / need to take a different approach, it's not a problem at all.

[rdblue]

Merged. Thanks, @kbendick! I used Rebase & Merge to add all of these to the branch as individual cherry-picked commits.

[kbendick]

Merged. Thanks, @kbendick! I used Rebase & Merge to add all of these to the branch as individual cherry-picked commits.

Great. Thank you @rdblue! I will make a PR (or two) with the remaining patches ASAP so we can cut a 0.12.1rc0.