multi.jl: refactor of worker management
 - fix/address comments by amitmurphy
 - avoid using Worker in manage(), .id and .config should suffice
 - force worker config to be passed around. ClusterManagers.jl has a pending update to accomodate this change
 - fix :cmd instance type in start_cluster_workers()
 - manage(:register) in a schedule block
Merge pull request #3926 from nlhepler/worker-work

multi.jl: simplification/refactor of worker management