Reorganize executor
Took @severo 's comments from https://github.com/huggingface/datasets-server/pull/827 and https://github.com/huggingface/datasets-server/pull/824

In particular:
- I moved the queue related stuff to queue.py
- I moved all the job runner related stuff to job_runner.py
- I shortened the env variable names
- I added Job.info() to avoid repeated the same error-prone dict creation over and over
- I added tests for each part that I moved
Reorganize executor: Took @severo 's comments from https://github.com/huggingface/datasets-server/pull/827 and https://github.com/huggingface/datasets-server/pull/824

In particular:
- I moved the queue related stuff to queue.py
- I moved all the job runner related stuff to job_runner.py
- I shortened the env variable names
- I added Job.info() to avoid repeated the same error-prone dict creation over and over
- I added tests for each part that I moved