Worker globals removal: _currentUserData

This PR:

- removes the `_currentUserData` global from the worker, and moves its logic to `tkWorker.user`
- renames `TkWorker#currentUsers` to `TkWorker#users`
- adds a `TkWorker#closeSession` to avoid logic duplication across the worker script code

Of course the worker refactoring doesn't end here, but small steps matter.

Suggested reviewer: anybody willing to.