Fix caching of /detail for modified reviewer lists

Rely only on ETag for change entity caching. The Last-Modified
value was derived from the lastUpdatedOn property, which is not
always modified when children are updated.

When adding or removing reviewers from a change perform a no-op
modification of the Change entity. This bumps its rowVersion as
a side-effect of the database mutation, causing the ETag to be
different on the next read.

Change-Id: If480f92b419041e7d62839ab12211058e78f828f

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

@@ -16,16 +16,12 @@ package com.google.gerrit.server.change;
 
 import com.google.gerrit.extensions.restapi.RestResource;
 import com.google.gerrit.extensions.restapi.RestResource.HasETag;
-import com.google.gerrit.extensions.restapi.RestResource.HasLastModified;
 import com.google.gerrit.extensions.restapi.RestView;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.server.project.ChangeControl;
 import com.google.inject.TypeLiteral;
 
-import java.sql.Timestamp;
-
-public class ChangeResource implements RestResource,
-    HasLastModified, HasETag {
+public class ChangeResource implements RestResource, HasETag {
   public static final TypeLiteral<RestView<ChangeResource>> CHANGE_KIND =
       new TypeLiteral<RestView<ChangeResource>>() {};
 
@@ -47,11 +43,6 @@ public class ChangeResource implements RestResource,
     return getControl().getChange();
   }
 
-  @Override
-  public Timestamp getLastModified() {
-    return getChange().getLastUpdatedOn();
-  }
-
   @Override
   public String getETag() {
     return String.format("%x-%x",