Regressions and potential bugs from #6363

[gjoseph92]

#6363 introduced a couple changes that may cause regressions or bugs. They are changes we want to make eventually, but don't have thorough enough testing to be sure about them and aren't strictly necessary for the original goals.

The main issues are:

• Switching Worker.close to restart the worker by default when there's a Nanny, rather than shutting down the Nanny
• Removing scheduler WorkerState before the worker is fully closed Remove report and safe from Worker.close #6363 (comment)

These will be done more thoroughly in

• Restart worker via Nanny on connection failure #6387
• Eliminate partially-removed-worker state on scheduler (comms open, state removed) #6390