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/map.rs

@@ -0,0 +1,99 @@
+use serde::{Deserialize, Serialize};
+
+use crate::crdt::crdt::*;
+
+/// Simple CRDT Map
+///
+/// This types defines a CRDT for a map from keys to values. Values are CRDT types which
+/// can have their own updating logic.
+///
+/// Internally, the map is stored as a vector of keys and values, sorted by ascending key order.
+/// This is why the key type `K` must implement `Ord` (and also to ensure a unique serialization,
+/// such that two values can be compared for equality based on their hashes). As a consequence,
+/// insertions take `O(n)` time. This means that Map should be used for reasonably small maps.
+/// However, note that even if we were using a more efficient data structure such as a `BTreeMap`,
+/// the serialization cost `O(n)` would still have to be paid at each modification, so we are
+/// actually not losing anything here.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Map<K, V> {
+	vals: Vec<(K, V)>,
+}
+
+impl<K, V> Map<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + Crdt,
+{
+	/// Create a new empty map CRDT
+	pub fn new() -> Self {
+		Self { vals: vec![] }
+	}
+
+	/// Returns a map that contains a single mapping from the specified key to the specified value.
+	/// This can be used to build a delta-mutator:
+	/// when merged with another map, the value will be added or CRDT-merged if a previous
+	/// value already exists.
+	pub fn put_mutator(k: K, v: V) -> Self {
+		Self { vals: vec![(k, v)] }
+	}
+
+	/// Add a value to the map
+	pub fn put(&mut self, k: K, v: V) {
+		self.merge(&Self::put_mutator(k, v));
+	}
+
+	/// Removes all values from the map
+	pub fn clear(&mut self) {
+		self.vals.clear();
+	}
+
+	/// Get a reference to the value assigned to a key
+	pub fn get(&self, k: &K) -> Option<&V> {
+		match self.vals.binary_search_by(|(k2, _)| k2.cmp(k)) {
+			Ok(i) => Some(&self.vals[i].1),
+			Err(_) => None,
+		}
+	}
+	/// Gets a reference to all of the items, as a slice. Usefull to iterate on all map values.
+	pub fn items(&self) -> &[(K, V)] {
+		&self.vals[..]
+	}
+	/// Returns the number of items in the map
+	pub fn len(&self) -> usize {
+		self.vals.len()
+	}
+
+	/// Returns true if the map is empty
+	pub fn is_empty(&self) -> bool {
+		self.len() == 0
+	}
+}
+
+impl<K, V> Crdt for Map<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + Crdt,
+{
+	fn merge(&mut self, other: &Self) {
+		for (k, v2) in other.vals.iter() {
+			match self.vals.binary_search_by(|(k2, _)| k2.cmp(k)) {
+				Ok(i) => {
+					self.vals[i].1.merge(v2);
+				}
+				Err(i) => {
+					self.vals.insert(i, (k.clone(), v2.clone()));
+				}
+			}
+		}
+	}
+}
+
+impl<K, V> Default for Map<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + Crdt,
+{
+	fn default() -> Self {
+		Self::new()
+	}
+}