send entire xdcr batch as single xmem batch

There's no need to try to split it because we have all the bodies
anyways. I.e. in 2.x we actually fetched bodies in replication worker,
but upr already sends us full docs so incoming batches are already
sized semi-properly.

Change-Id: Ia4e84b328be4ce5cf82fc94a3a7bcf25f8481a73
Reviewed-on: http://review.couchbase.org/40093
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>