Get new snapshot of "stream bandwidth by percentiles"

Check out Figure 10 of Mike's original torflow paper:
https://research.torproject.org/techreports/torflow-2009-08-07.pdf

In that paper, Mike found that the fastest relays in the network were providing better bandwidth to clients than the slower relays. Whereas in an ideal world, the load on the network should be balanced such that every relay offers the same experience (because every relay has the appropriate load).

The world has moved on a lot from 2009, including newer bwauths and impacts from many other network dynamics.

We have plans in the future to make even more improvements, for example with Mike's upcoming flow control changes.

Let's:

• (a) get a baseline now, and
• (b) get the tools together to enable us to more easily get more snapshots as we proceed.