Manager shutdown after inactivity

[dgasmith]

Managers should have the option to shutdown after x period of inactivity. Where this is defined as an empty task queue for the given period, a new task will reset this timer. By default this period should be 0 (or -1) to indicate that the manager will not shutdown after a period activity.

A few use cases for this:

• Shut down forgotten managers after multiple days of inactivity and for users who do want to leave managers spun up indefinitely.
• Shut down process pool managers to prevent wasting resources as they are unable to dynamically scale their workers.