Node candidates should be checked during redeployment

[rudi]

Currently, during redeployment we only ask the CFSB broker for node candidates for a component when either the component requirements or the replica count have changed; in the latter case, we do not reconsider placement of existing replicas.

Since node candidate rankings might have changed, ask for the ranked node candidates for all components during redeployment, and redeploy the component if the ranking changed.

[rudi]

This also handles the case of vanishing edge nodes -- these will trigger a redeployment. If we check the status of all already-deployed components, the component(s) running on the crashed edge node will be redeployed.

[rudi]

Note: make sure that CFSB still tells us about an edge node candidate that is busy, otherwise we will assume the node has vanished and will redeploy away from it.