Improved opentracing support

[erwinvaneyk]

This PR improves the opentracing support in workflows in multiple ways:

• it adds a couple of additional spans to trace: controller evaluations, fission function calls, workflow calls.
• it adds logs to spans, which include (in debug mode) all inputs, outputs and expressions.
• it adds tags to the spans, including the function references, workflow ids, task ids, and statuses, to enable searching for specific tasks.
• Adds documentation on how to use opentracing for local development.