Check for locked after unlocked

[colin-grant-work]

What it does

Fixes #10824 by checking whether the queue is in use before disposing of it. Apparently new actions could be enqueued between the resolution of the watiForUnlock promise and the actual running of the wait for unlick code.

How to test

1. I have added a test that ensures that preference transactions can handle many synchronous settings: observe that that passes.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[colin-grant-work]

@msujew, I believe that this fixes the problems with multiple preference settings occurring synchronously. It also appears to fix the message about the file being dirty, although it doesn't directly address it - all of my attempts to address it directly have only made things worse, so I've left them out of this PR.

[msujew]

Thanks Colin, works great!

• Setting multiple preferences in parallel correctly updates all of them
• Fixes the errors/issues experienced when quickly changing themes
• The test passes