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.

1. 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.