Race condition when spawning git processes for autograding

[meffmadd]

Describe the bug
The bug is caused by the tornado Subprocess.wait_for_exit class: tornadoweb/tornado#3364

Grader-Service/grader_service/autograding/local_grader.py

Line 440 in baaaca0

await process.wait_for_exit()

I noticed this when testing autograding in the Celery workers where it would hang on git init.

This could also affect the git server:

Grader-Service/grader_service/handlers/git/server.py

Line 48 in baaaca0

IOLoop.current().spawn_callback(self.process.wait_for_exit)

I switched to Popen for the Celery branch, which is a drop-in replacement.