CBD-836: add multiset/multiget xdcr lag stats

Change-Id: I2c478a40c9086cc3d907326e84f9e58995929eee
Reviewed-on: http://review.couchbase.org/24545
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>