added ability to show averaged results

Again multiple runs on the same spec:
- instead of showing all results, sometimes we want them to be averaged.

In order to give more control to application code, choose not to use
sql database's avg function.

Change-Id: I43c815ef6b8973765c5917813c942a18aa92f1f2
Reviewed-on: http://review.couchbase.org/23623
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>