change: skip cp lookup on no change

Modify `Change.rectify` to look up the current checkpoint only when
there was actually some change made.

This should improve the performance of `rectifyAll` when called from a
regular timer and there were no changes made since the last call.
Before this commit, `rectifyAll` would perform N calls of
`Checkpoint.current` where N is the number of model instances. With
this commit in place, no call is made.

common/models/change.js

@@ -175,24 +175,31 @@ module.exports = function(Change) {
       if (err) throw new Error(err);
     };
 
-    async.parallel([
+    change.currentRevision(function(err, rev) {
-      function getCurrentCheckpoint(next) {
-        change.constructor.getCheckpointModel().current(next);
-      },
-      function getCurrentRevision(next) {
-        change.currentRevision(next);
-      }
-    ], doRectify);
-
-    function doRectify(err, results) {
       if (err) return cb(err);
-      var checkpoint = results[0];
-      var rev = results[1];
 
+      // avoid setting rev and prev to the same value
+      if (currentRev === rev) {
+        change.debug('rev and prev are equal (not updating anything)');
+        return cb(null, change);
+      }
+
+      // FIXME(@bajtos) Allo callers to pass in the checkpoint value
+      // (or even better - a memoized async function to get the cp value)
+      // That will enable `rectifyAll` to cache the checkpoint value
+      change.constructor.getCheckpointModel().current(
+        function(err, checkpoint) {
+          if (err) return cb(err);
+          doRectify(checkpoint, rev);
+        }
+      );
+    });
+
+    function doRectify(checkpoint, rev) {
       if (rev) {
-        // avoid setting rev and prev to the same value
         if (currentRev === rev) {
-          change.debug('rev and prev are equal (not updating anything)');
+          change.debug('ASSERTION FAILED: Change currentRev==rev ' +
+            'should have been already handled');
           return cb(null, change);
         } else {
           change.rev = rev;