CBD-792: use smarter sampling interval in xdcr lag stats

Change-Id: I058cb58d545b53a414aa5f25cca878e7ace0d404
Reviewed-on: http://review.couchbase.org/24270
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>