MB-7275: fix XDCR aggregated stats issue

Today xdcr checkpoints when a new vb replicator starts but we did
not account the commit time into the stats. We also persist incorrect
number of docs checked/written and data replicated into checkpoint
doc. This change-set will fix the two issues above.

Change-Id: I62e4682871dcfb020dff5e04538be07b911d0a34
Reviewed-on: http://review.couchbase.org/22864
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>