Ensure TaskState instances are released on Scheduler and Worker

[fjetter]

There are strong references on our TaskState instances, Scheduler and Worker instances

See for instance #6246

We should ensure that TaskState is are properly cleaned up

• There is a test in Scheduler.validate that ensure TaskState instances are properly released Ensure scheduler events do not hold on to TaskState objects #6226 (comment)
• utils_test.check_instances verifies that there are no leftover Scheduler, Worker, Nanny or TaskState instances

Caveat: This depends on the profiler thread so we may need to relax the above tests and replace them with a more robust, less strict version as appropriate

#6246

• Ensure garbage collection of Nanny instances #6336
• Ensure garbage collection of Worker instances #6344
• Ensure garbage collection of Scheduler instances #6345

[sjperkins]

This PR address Scheduler instance leaks

• Garbage collect Scheduler Instances during testing #6281

[gjoseph92]

I don't believe this was fully closed; #6525 was just the first step to being able to track it.