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.

src/rpc/rpc_helper.rs

@@ -225,7 +225,7 @@ impl RpcHelper {
 			// Retrieve some status variables that we will use to sort requests
 			let peer_list = self.0.fullmesh.get_peer_list();
 			let ring: Arc<Ring> = self.0.ring.borrow().clone();
-			let our_zone = match ring.config.members.get(&self.0.our_node_id) {
+			let our_zone = match ring.layout.node_role(&self.0.our_node_id) {
 				Some(pc) => &pc.zone,
 				None => "",
 			};
@@ -238,7 +238,7 @@ impl RpcHelper {
 			// and within a same zone we priorize nodes with the lowest latency.
 			let mut requests = requests
 				.map(|(to, fut)| {
-					let peer_zone = match ring.config.members.get(&to) {
+					let peer_zone = match ring.layout.node_role(&to) {
 						Some(pc) => &pc.zone,
 						None => "",
 					};