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.

README.md

@@ -3,10 +3,18 @@ Garage [![Build Status](https://drone.deuxfleurs.fr/api/badges/Deuxfleurs/garage
 
 <p align="center" style="text-align:center;">
 	<a href="https://garagehq.deuxfleurs.fr">
-	<img alt="Garage logo" src="doc/logo/garage.png" height="200" />
+	<img alt="Garage logo" src="https://garagehq.deuxfleurs.fr/img/logo.svg" height="200" />
 	</a>
 </p>
 
+<p align="center" style="text-align:center;">
+	[ <strong><a href="https://garagehq.deuxfleurs.fr/">Website and documentation</a></strong>
+	| <a href="https://garagehq.deuxfleurs.fr/_releases.html">Binary releases</a>
+	| <a href="https://git.deuxfleurs.fr/Deuxfleurs/garage">Git repository</a>
+	| <a href="https://matrix.to/#/%23garage:deuxfleurs.fr">Matrix channel</a>
+	]
+</p>
+
 Garage is a lightweight S3-compatible distributed object store, with the following goals:
 
 - As self-contained as possible
@@ -22,5 +30,3 @@ Non-goals include:
 - Erasure coding (our replication model is simply to copy the data as is on several nodes, in different datacenters if possible)
 
 Our main use case is to provide a distributed storage layer for small-scale self hosted services such as [Deuxfleurs](https://deuxfleurs.fr).
-
-**[Go to the documentation](https://garagehq.deuxfleurs.fr)**