Add traffic metering to locate

[mattmathis]

New feature that can be used to limit test rates to specific sites, by making a site ineligible to receive another test for a selectable number of seconds, whenever it is returned in a locate result. This is analogous to metering lights at highway entrance ramps.

This can be used to gather a sip of data from a resource constrained server or site. For example with the timer set to 30 seconds we might get 500-3000 non-overlapping tests per day, (depending on the number of other servers returned by locate). The site would not need to be able to support concurrent tests, but it would give us enough data to detect network anomalies that might justify deploying a more expensive server.

This should be considered in conjunction with #92.