Client should iterate over Locate API results

[stephen-soltesz]

As currently written, the ndt7-js client uses a random result returned by the Locate API and does not save the unused results. This is a mistake. Because any ndt-server may decline to run a measurement, the client should iterate through available results from the Locate API until a working server is found or all results are exhausted.

[nkinkade]

Another unintended consequence of selecting a random site from the results returned from the locate service is that singleton sites will have a high probability of testing to a server that is neither close, nor necessarily even in the same country. A good example of this is at NBO01 in Kenya. Currently, if you query the locate service like this:

http://locate.measurementlab.net/v2/nearest/ndt/ndt7?country=KE

... you get something like:

{
  "results": [
    {
      "machine": "mlab3-nbo01.mlab-oti.measurement-lab.org",
      <snip>
    },
    {
      "machine": "mlab1-tnr01.mlab-oti.measurement-lab.org",
      <snip>
    },
    {
      "machine": "mlab2-jnb01.mlab-oti.measurement-lab.org",
      <snip>
    },
    {
      "machine": "mlab2-los02.mlab-oti.measurement-lab.org",
      <snip>
    }
  ]
}

This means that there is a 75% chance that client will test to a location quite far away and in a different country.

A quick remedy for this would be to stop selecting targets at random and to iterate through them from first to last. Notwithstanding other issues, this would give a client a good probability of running a successful test to the closest server.