4380: Node Manager monitors respond to shutdown_eligible message.
4380: Node Manager shutdown actor is more robust.

ComputeNodeShutdownActor now checks that destroying the cloud node
succeeds.  Before retrying, it will check if the node is still
eligible for shutdown, and abort if not.
4380: Fix retry scheduling for Node Manager change methods.
4380: Add SLURM dispatcher to Node Manager.
Merge branch '4380-node-manager-slurm-drain-wip'

Closes #4380, #4519.