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/util/crdt/bool.rs

@@ -0,0 +1,34 @@
+use serde::{Deserialize, Serialize};
+
+use crate::crdt::crdt::*;
+
+/// Boolean, where `true` is an absorbing state
+#[derive(Clone, Copy, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Bool(bool);
+
+impl Bool {
+	/// Create a new boolean with the specified value
+	pub fn new(b: bool) -> Self {
+		Self(b)
+	}
+	/// Set the boolean to true
+	pub fn set(&mut self) {
+		self.0 = true;
+	}
+	/// Get the boolean value
+	pub fn get(&self) -> bool {
+		self.0
+	}
+}
+
+impl From<bool> for Bool {
+	fn from(b: bool) -> Bool {
+		Bool::new(b)
+	}
+}
+
+impl Crdt for Bool {
+	fn merge(&mut self, other: &Self) {
+		self.0 = self.0 || other.0;
+	}
+}