Misleading store state

Currently, the status of down/disconnect is not reflected in the meta information of the store. As long as you do not manually go offline, the meta is always Up. This leads to the inability to determine the down state when using the grpc or HTTP interface to query the meta information of the store, and it is easy to mistake it as normal (but not).

Proposal: Define new state to use in the API, distinguish from that persistent to storage.

Start by tracing how store status and meta information are represented and exposed through the gRPC and HTTP interfaces. Define done as an API-visible state that distinguishes a down or disconnected store from a store that is persistently marked offline, without persisting the new state.

Written by the indexing model from the issue text.