Remove GetOptionsDiagnostics, global diags in editor, restore checker affinity

[jakebailey]

This builds on #2322. Probably could replace it and just go for this.

From #2322:

• Remove GetOptionsDiagnostics
• Expose global diags to the editor.

Then:

• Restore the looping over checkers to walk files in a consistent order and keep them in the same goroutine during many file checking.
• Refactor the CheckerPool interface again, now down to just one single function.
  • As a side effect, fixes a bug where the project pool was not giving requests a consistent checker.