Opentracing support

Parseq tracing is great. I think it would be great to see it adopt the [opentracing](http://opentracing.io) standard. Opentracing is heavily used by outside of LinkedIn and there is support for much of the tech used by LinkedIn including Couchbase, Play Framework, MongoDB, and Datadog (the latter two are used by Connectifier).

[Documentation on instrumenting frameworks with opentracing](http://opentracing.io/documentation/pages/instrumentation/instrumenting-frameworks.html)
[Java opentracing library](https://github.com/opentracing/opentracing-java)

The issue does not name files, tests, or a concrete integration point. Start by reviewing Parseq's existing tracing support and the linked OpenTracing instrumentation documentation and Java library. Before implementation, agree on the supported scope and acceptance criteria; done would mean that agreed Parseq tracing behavior is exposed through OpenTracing.

Written by the indexing model from the issue text.