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/web/Cargo.toml

@@ -1,11 +1,12 @@
 [package]
 name = "garage_web"
-version = "0.4.0"
+version = "0.5.0"
 authors = ["Alex Auvolat <alex@adnab.me>", "Quentin Dufour <quentin@dufour.io>"]
 edition = "2018"
 license = "AGPL-3.0"
 description = "S3-like website endpoint crate for the Garage object store"
 repository = "https://git.deuxfleurs.fr/Deuxfleurs/garage"
+readme = "../../README.md"
 
 [lib]
 path = "lib.rs"
@@ -13,10 +14,10 @@ path = "lib.rs"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-garage_api = { version = "0.4.0", path = "../api" }
-garage_model = { version = "0.4.0", path = "../model" }
-garage_util = { version = "0.4.0", path = "../util" }
-garage_table = { version = "0.4.0", path = "../table" }
+garage_api = { version = "0.5.0", path = "../api" }
+garage_model = { version = "0.5.0", path = "../model" }
+garage_util = { version = "0.5.0", path = "../util" }
+garage_table = { version = "0.5.0", path = "../table" }
 
 err-derive = "0.3"
 log = "0.4"