add vtbenchstat

[systay]

Adds vtbenchstat, a binary to summarize or compare trace logs

Part of #21

Here is an example. I created a special build of Vitess with one of the planner rules disabled, and ran the tpch test with the normal build and the special build.

./vtbenchstat without-pushing.json with-pushing.json

Query: select c_name, c_custkey, o_orderkey, o_orderdate, o_totalprice, sum(l_quantity) from customer, orders, lineitem where o_orderkey in ( select l_orderkey from lineitem group by l_orderkey having sum(l_quantity) > 314 ) and c_custkey = o_custkey and o_orderkey = l_orderkey group by c_name, c_custkey, o_orderkey, o_orderdate, o_totalprice order by o_totalprice desc, o_orderdate limit 100;
Line # 1105
+----------------+----------------------+-------------------+------+----------+
|     Metric     | without-pushing.json | with-pushing.json | Diff | % Change |
+----------------+----------------------+-------------------+------+----------+
| Route Calls    |                  204 |                14 | -190 | -93.14%  |
| Rows Sent      |                  410 |                30 | -380 | -92.68%  |
| Rows In Memory |                  100 |                 5 |  -95 | -95.00%  |
+----------------+----------------------+-------------------+------+----------+

...


Summary:
- 4 out of 22 queries showed significant change
- Average change in Route Calls: -11.71%
- Average change in Data Sent: -11.79%
- Average change in Rows In Memory: -7.35%