Unified state management for device/source/subscriptions actors

[ddjoh]

For main->device->source->subscription tree actors:

• Unified state management through mixin-ish classes (until actor subtyping support is added to acton)
• Start when upper level has successfully intialized any required resources (e.g. source protocol connection)
• At stop/close each level await lower level actors to complete stop/close.
• New actors reusing the configuration key (e.g. device/source/subscription name) of a previously removed/closed actor hold off startup pending the completed closing (e.g. sink cleanup) of it's predecessor

Reset (cleanup) notification for subscription->transform->sink chains:

• Optional callback to allow awaiting e.g. sink cleanup operations before closing the originating source/subscriber