Use ETag and If-None-Match for /detail caching

Last-Modified is only second level precision and may allow a client to
fail to see subsequent updates within the same second window.  Improve
the caching check by using an ETag string that is computed from the
full timestamp and the row version fields of Change.

Change-Id: I58fc95b5396baa1d8afb676ff672c9f19b835f1c

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/RestResource.java

@@ -26,6 +26,15 @@ public interface RestResource {
 
   /** A resource with a last modification date. */
   public interface HasLastModified {
+    /**
+     * @return time for the Last-Modified header. HTTP truncates the header
+     *         value to seconds.
+     */
     public Timestamp getLastModified();
   }
+
+  /** A resource with an ETag. */
+  public interface HasETag {
+    public String getETag();
+  }
 }