Throttle rate of Zipkin error messages

[Aaronontheweb]

Happens after generating a couple hundred spans per second for 10-15 minutes at a time while running Zipkin locally in a stand-alone Docker for Windows container with no backing store, but eventually we can tip the system over without a huge amount of effort.

In those cases, we should probably limit the number of log messages we display when connectivity to Zipkin starts to backfire.

Related to #27