Parse and serve reviewer updates via REST API

Parses updates to a change's reviewers, and serves them with the change
details. Will be picked up by PolyGerrit UI and displayed in change's
messages section in follow-up change.

Feature availability depends on NoteDb migration.

Feature: Issue 4148
Change-Id: I50e75ba9e5a885de4733a3a8d7d156d3729d1d91

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index aa51a92..e1e6b44 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -264,6 +264,12 @@
   fields when referencing accounts.
 --
 
+[[reviewer-updates]]
+--
+* `REVIEWER_UPDATES`: include updates to reviewers set as
+  link:#review-update-info[ReviewerUpdateInfo] entities.
+--
+
 [[messages]]
 --
 * `MESSAGES`: include messages associated with the change.
@@ -511,8 +517,8 @@
 --
 
 Retrieves a change with link:#labels[labels], link:#detailed-labels[
-detailed labels], link:#detailed-accounts[detailed accounts], and
-link:#messages[messages].
+detailed labels], link:#detailed-accounts[detailed accounts],
+link:#reviewer-updates[reviewer updates], and link:#messages[messages].
 
 Additional fields can be obtained by adding `o` parameters, each
 option requires more database lookups and slows down the query
@@ -656,6 +662,56 @@
         }
       ]
     },
+    "reviewer_updates": [
+      {
+        "state": "REVIEWER",
+        "reviewer": {
+          "_account_id": 1000096,
+          "name": "John Doe",
+          "email": "john.doe@example.com",
+          "username": "jdoe"
+        },
+        "updated_by": {
+          "_account_id": 1000096,
+          "name": "John Doe",
+          "email": "john.doe@example.com",
+          "username": "jdoe"
+        },
+        "updated": "2016-07-21 20:12:39.000000000"
+      },
+      {
+        "state": "REMOVED",
+        "reviewer": {
+          "_account_id": 1000096,
+          "name": "John Doe",
+          "email": "john.doe@example.com",
+          "username": "jdoe"
+        },
+        "updated_by": {
+          "_account_id": 1000096,
+          "name": "John Doe",
+          "email": "john.doe@example.com",
+          "username": "jdoe"
+        },
+        "updated": "2016-07-21 20:12:33.000000000"
+      },
+      {
+        "state": "CC",
+        "reviewer": {
+          "_account_id": 1000096,
+          "name": "John Doe",
+          "email": "john.doe@example.com",
+          "username": "jdoe"
+        },
+        "updated_by": {
+          "_account_id": 1000096,
+          "name": "John Doe",
+          "email": "john.doe@example.com",
+          "username": "jdoe"
+        },
+        "updated": "2016-03-23 21:34:02.419000000",
+      },
+    ],
     "messages": [
       {
         "id": "YH-egE",
@@ -4316,6 +4372,11 @@
 `REMOVED`: Users that were previously reviewers on the change, but have
 been removed. +
 Only set if link:#detailed-labels[detailed labels] are requested.
+|`reviewer_updates`|optional|
+Updates to reviewers set for the change as
+link:#review-update-info[ReviewerUpdateInfo] entities.
+Only set if link:#reviewer-updates[reviewer updates] are requested and
+if NoteDb is enabled.
 |`messages`|optional|
 Messages associated with the change as a list of
 link:#change-message-info[ChangeMessageInfo] entities. +
@@ -5019,6 +5080,26 @@
 voting values.
 |===========================
 
+[[review-update-info]]
+=== ReviewerUpdateInfo
+The `ReviewerUpdateInfo` entity contains information about updates to
+change's reviewers set.
+
+[options="header",cols="1,6"]
+|===========================
+|Field Name     |Description
+|`updated`|
+Timestamp of the update.
+|`updated_by`|
+The account which modified state of the reviewer in question as
+link:rest-api-accounts.html#account-info[AccountInfo] entity.
+|`reviewer`|
+The reviewer account added or removed from the change as an
+link:rest-api-accounts.html#account-info[AccountInfo] entity.
+|`state`|
+The reviewer state, one of `REVIEWER`, `CC` or `REMOVED`.
+|===========================
+
 [[review-input]]
 === ReviewInput
 The `ReviewInput` entity contains information for adding a review to a