CBQE-0: rename self._num_items-> self.num_items for XDCR

it's difficult handle both params for mixed tests like XDCRViewTests
when we call verification methods from basetestcase and xdcrbasetestcase

Change-Id: I5e97bbcd7e099dd5b09ba8ccd2663ba8d959f813
Reviewed-on: http://review.couchbase.org/33253
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>