Write label names instead of IDs to PatchSetApprovals

This change only exists to allow online migration of
PatchSetApproval's categoryId in a live cluster deployment. To
migrate, we need to:
 - Update all servers to understand either label IDs or names in
   PatchSetApprovals (done prior to this change), while still writing
   label IDs (since during the rolling update, old servers continue to
   read/write only IDs).
 - Update all servers to write label names, while still understanding
   either IDs or names (this change).
 - Migrate live data in all PatchSetApprovals.
 - Delete all code related to label IDs and update servers (after this
   change).

Change-Id: I96d714de7b084634dfa9d1c6b5bb480f6868c354

gerrit-common/src/main/java/com/google/gerrit/common/data/LabelType.java

@@ -251,7 +251,7 @@ public class LabelType {
   }
 
   public LabelId getLabelId() {
-    return new LabelId(id != null ? id : name);
+    return new LabelId(name);
   }
 
   @Override