Improve how node roles are assigned in Garage

- change the terminology: the network configuration becomes the role
  table, the configuration of a nodes becomes a node's role
- the modification of the role table takes place in two steps: first,
  changes are staged in a CRDT data structure. Then, once the user is
  happy with the changes, they can commit them all at once (or revert
  them).
- update documentation
- fix tests
- implement smarter partition assignation algorithm

This patch breaks the format of the network configuration: when
migrating, the cluster will be in a state where no roles are assigned.
All roles must be re-assigned and commited at once. This migration
should not pose an issue.

script/dev-cluster.sh

@@ -69,7 +69,7 @@ done
 sleep 3
 # Establish connections between nodes
 for count in $(seq 1 3); do
-	NODE=$(garage -c /tmp/config.$count.toml node-id -q)
+	NODE=$(garage -c /tmp/config.$count.toml node id -q)
 	for count2 in $(seq 1 3); do
 		garage -c /tmp/config.$count2.toml node connect $NODE
 	done