BatchUpdate: Add more helper methods

Add new helper methods:

- getAccount
- getAccountId
- getIdentifiedUser

which will reduce the amount of code at call sites, for example:

  ctx.getAccount();

instead of:

  ctx.getUser().asIdentifiedUser().getAccount();

Change-Id: I946002e1cf70a77bc1fe3879edf5717ecac77fb7

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/notedb/ChangeRebuilderIT.java

@@ -407,7 +407,7 @@ public class ChangeRebuilderIT extends AbstractDaemonTest {
           PatchSet.Id psId = ctx.getChange().currentPatchSetId();
           ChangeMessage cm = new ChangeMessage(
               new ChangeMessage.Key(id, ChangeUtil.messageUUID(ctx.getDb())),
-                  ctx.getUser().getAccountId(), ctx.getWhen(), psId);
+                  ctx.getAccountId(), ctx.getWhen(), psId);
           cm.setMessage(msg);
           ctx.getDb().changeMessages().insert(Collections.singleton(cm));
           ctx.getUpdate(psId).setChangeMessage(msg);