BatchUpdate: Bump lastUpdatedOn by default

There are only two cases where we don't currently bump lastUpdatedOn:
adding reviewers and adding draft comments. The vast majority of other
cases we do want to bump lastUpdatedOn.

Change the ChangeContext#saveChange method to #bumpLastUpdatedOn, to
be explicit about what this actually does. Default to true to make it
easier on callers.

Change-Id: I3d8b13f70b082f8f4ef15df02f72840ed42eed85

gerrit-server/src/main/java/com/google/gerrit/server/change/PutTopic.java

@@ -114,7 +114,6 @@ public class PutTopic implements RestModifyView<ChangeResource, Input>,
       }
       change.setTopic(Strings.emptyToNull(newTopicName));
       update.setTopic(change.getTopic());
-      ctx.saveChange();
 
       ChangeMessage cmsg = new ChangeMessage(
           new ChangeMessage.Key(