Replication
-----------

- for each interval of tokens, we know the list of nodes that are responsible
- every node watches the current ring and state of the network
- and thus determines the interval of tokens for which they are responsible



To do list
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- important: check block values on read and repare corrupted block contents
- less a priority: hinted handoff
- FIXME in rpc_server when garage shuts down and futures can be interrupted
  (tokio::spawn should be replaced by a new function background::spawn_joinable)