Feature/update process tracer

Update **`ZipkinTracer::ProcessTracer`** to rely on **`ZipkinTracer::IntraProcessTraceId`** instead of explicitly calling **`Thread`**. Changes:
* Add private getter methods for the current ZipkinTracer::IntraProcessTraceId's trace and span ids to ZipkinTracer::ProcessTracer and use these methods in internal tracing methods #trace_internal and #trace_child.
* Set the current IntraProcessTraceId to the new trace id as first method call in #record, right before #push
* Modify specs accordingly

All specs passing:
```
~/Projects/mdsol_services/zipkin-tracer [feature/update_process_tracer|✔] 
$ bundle exec rspec spec/
...

Finished in 2.24 seconds
45 examples, 0 failures
```

Also adhoc tested this gem with Archon integration and is working as expected (if interested, can review WIP branch in Archon repo [feature/upgrade_zipkin_tracer](https://github.com/mdsol/Archon/compare/feature;upgrade_zipkin_tracer))