Merge changes I67ea6014,Iac484155,I1cf3aafd

* changes:
  Add the groups external cache to the config-gerrit documentation
  Split the groups external cache into 2 caches for better performance
  Add a test method for the groups external cache

proto/cache.proto

@@ -270,6 +270,15 @@ message AllExternalIdsProto {
   repeated ExternalIdProto external_id = 1;
 }
 
+// Serialized form of a list of com.google.gerrit.entities.AccountGroup.UUID
+// Next ID: 2
+message AllExternalGroupsProto {
+  message ExternalGroupProto {
+    string groupUuid = 1;
+  }
+  repeated ExternalGroupProto external_group = 1;
+}
+
 // Key for com.google.gerrit.server.git.PureRevertCache.
 // Next ID: 4
 message PureRevertKeyProto {