CBD-792: plot xdcr lag diff time

Change-Id: I105353f7a9d78b40601dbaaaf365b6c01d67137b
Reviewed-on: http://review.couchbase.org/24271
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>