Use the opentracing support in go-pilosa

[jaffee]

Need to expose options so that PDK ingestion code can configure go-pilosa to use tracing.

Additionally, the usual configuration will probably be that go-pilosa is writing to the same tracing infra as Pilosa, so it might be worth Pilosa exposing its tracing config through an endpoint which go-pilosa can consume, thereby allowing hands-off config by default... 🤔 This second part is not too relevant directly to the PDK... I think we still need the first part anyway.