Fix test race condition, and add Datadog event support

[wschroeder]

This PR includes two different things:

• When running mix test, I saw intermittent failures because many tests attempted to start a server that listened on the same port. This is resolved by making each test server start on a unique port.
• I brought in milmazz's addition of Datadog events, and then I fixed the line protocol bug for the feature: Datadog's events do not start with a prefix. This required a much-needed change that separated the concerns of the port_command header (destination information) from the UDP data (prefix and then the rest of the metric name and line protocol).

[cpiemontese]

It would be really cool to have event support! Any chance this will be merged?