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/table/lib.rs

@@ -4,7 +4,6 @@
 #[macro_use]
 extern crate log;
 
-pub mod crdt;
 pub mod schema;
 pub mod util;
 
@@ -18,3 +17,7 @@ pub mod table;
 pub use schema::*;
 pub use table::*;
 pub use util::*;
+
+pub mod crdt {
+	pub use garage_util::crdt::*;
+}