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.

Cargo.lock

@@ -379,7 +379,7 @@ dependencies = [
 
 [[package]]
 name = "garage"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "async-trait",
  "bytes 1.1.0",
@@ -408,7 +408,7 @@ dependencies = [
 
 [[package]]
 name = "garage_api"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "base64",
  "bytes 1.1.0",
@@ -440,7 +440,7 @@ dependencies = [
 
 [[package]]
 name = "garage_model"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "arc-swap",
  "async-trait",
@@ -462,7 +462,7 @@ dependencies = [
 
 [[package]]
 name = "garage_rpc"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "arc-swap",
  "async-trait",
@@ -479,6 +479,7 @@ dependencies = [
  "rand",
  "rmp-serde 0.15.5",
  "serde",
+ "serde_bytes",
  "serde_json",
  "tokio",
  "tokio-stream",
@@ -486,7 +487,7 @@ dependencies = [
 
 [[package]]
 name = "garage_table"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "async-trait",
  "bytes 1.1.0",
@@ -506,7 +507,7 @@ dependencies = [
 
 [[package]]
 name = "garage_util"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "blake2",
  "chrono",
@@ -530,7 +531,7 @@ dependencies = [
 
 [[package]]
 name = "garage_web"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "err-derive 0.3.0",
  "futures",