refactor: split job.py and test_job.py

[tswast]

job.py and test_job.py are both thousands of lines long. It's actually causing my code editor to slow down. These files have a natural split into the various job types:

• _base -- for the _AsyncJob base class
• extract
• copy
• load
• query

The job/__init__.py should import all of these sub-modules to retain backwards compatibility.

Might want to wait for #41 and #347 to be merged before working on this.