Dacha1 - Can be shut down mid-cache update broadcast

[rvowles]

In a situation when a deployment is rolling (say in k8s), a dacha1 instance can be shut down while it is in the middle of handing its cache off to a new dacha instance. This can lead to confusion over the state and potentially lead to cache corruption.

Acceptance criteria:

• the dacha instance should refuse to shut down until it has finishes broadcasting its dacha if it has started

• the front end should allow the user to refresh the cache if a superuser and the app is using the Dacha1 cache.

• potentially expose the status of each Dacha1 cache discovered on the UI.