[CX] Rebalance when DCP states are different

Currently it is possible for a rebalance of Analytics nodes to always fail if

• a bucket is in the disconnected state and
• the shadow datasets are at different DCP states.

In that case a common DCP state (required for rebalancing) cannot be achieved as no new DCP mutations will be received by the Analytics nodes.

We should allow a user to handle or to avoid this situation. E.g. we could

• ensure that the DCP states are always consistent when (gracefully) disconnecting or we could
• skip datasets with different DCP states during rebalancing or we could
• have a mechanism to catch up to a consistent state to enable rebalance (which might be triggered manually or automatically by a rebalance) or we could
• rebalance in spite of the inconsistent state and reingest from 0 on reconnection.