CBQE-0: simplify _verify_data_all_buckets and _wait_flusher_empty

Change-Id: I0b9a013c7fbe9721f1fe2b99ec1fe7d5c479478e
Reviewed-on: http://review.couchbase.org/41075
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>