CBD-792: add measurements for xdcr lags

Change-Id: I99035511f22b4b4d4108a6726af9e2c4a7073577
Reviewed-on: http://review.couchbase.org/24247
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: buildbot <build@couchbase.com>