Validate that our clients can measure common link rates in underdeveloped areas

[mattmathis]

We should go as low a 56kbit/s, which is one voice grade telephony circuit (7 bits * 8 kHz). Ideally we could go as low as old screechy modems (24 kbit/s or 12 kbit/s) but that might not be practical.

Note that this is only 70 kBytes in 10 seconds. If we start at 64kB buffers, we might be ok.

[robertodauria]

m-lab/ndt7-client-go#75 should be a step forward in this direction -- sender-side measurements are usually overestimated, and this gets worse on slower links. The impact on a 56kbit/s is probably very significant, as almost a whole 64kB message would be in the send buffer at the end of 10s.