Instrument using custom attributes + add Worker tests

[hammady]

This refactors the Worker class with the below notable changes:

1. It now reports transaction custom attributes to Newrelic instead of custom metrics
2. Both transient and permanent errors (failures) are now reported as Boolean attributes
3. Major functions are now fully tested, also with a new GitHub action to run on PRs and master branch pushes

Other changes:

1. The worker does not sleep between handling jobs. It only sleeps when no jobs are found on the queue
2. Unsupported jobs are now instrumented
3. Job handling logic is extracted out in a new function to simplify the worker infinite running loop

Future work:

1. Report enqueue time attributes
2. Introduce a simple way for applications using this library to report more custom attributes
3. Test other classes
4. Report test coverage
5. Cherry-pick commits to support python2.7 in the corresponding branch