XDCR: need to check stats before verification of all buckets

Today in xdcr tests testrunner, we see code like below, which verify buckets before verify all stats.

      # CHecking stats only on source-1 , sice that has the last update |
            self._verify_all_buckets(src_master) |
            self._verify_stats_all_buckets(src_nodes)

This may not be correct since we need to verify the stats first to make sure replication is done, and then verify the data in bucket.