roachtest: add impact tests

[andrewbaptist]

Adds a new latency sensitive test and a framework to run them in. Specifically it does a few things:

1. Gathers all the latency results back at the "roachtest client" rather than at the workload nodes by utilizing UDP to aggregate the results.
2. Steps the load back to 30% of the original load by measuring during the fill and then using that as a baseline to calculate from.
3. Collects selective profiles and adds them to the artifacts as part of the test run.

As of today, this test doesn't fail in the case of high latency as I'm working out the details of where to draw the threshold, but it has all the data so we can make that decision once we have more of these tests written.