API: provide list of shuttle pings, aggregated by SP

[jcace]

Provide a method for the shuttle pings to be queried from the API, based on SP ID. This will be very valuable for troubleshooting and for SPs to know how their latency looks like to each of our shuttles. If SPs are not getting as many deals as expected, this will be a key indicator they can look at to determine why.

For example.
/get/sp-selection/f01234

returns

[
  {
    "shuttle": "shuttle-1",
    "ping": 68.12
  },
  {
    "shuttle": "shuttle-2",
    "ping": 106.23
  }
]

[jcace]

Might make sense to cache the entire list of all shuttle <-> SP pings on the API so that there's a nice central place to look them up.