MB-100: changed cbr test to have 50M items

Change-Id: I6d91f25b0c839837abdb162ba0df234dd7f157e8
Reviewed-on: http://review.couchbase.org/26326
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>