[WIP][AIRFLOW-2761] Parallelize enqueue in celery executor

[KevinYang21]

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Summary of major changes:
• Celery executor will create a process pool to enqueue celery task in parallel.
• Scheduler job will not pass TaskInstance to executor when trying to queue command, instead it passes SimpleTaskInstance, in order to enable parallel enqueuing.
• Scheduler no longer checks task instance state before enqueuing because worker already checks task instance state before executing. Our cluster has been requeuing everything for a long time and there should be no risk removing that redundant check.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

tests/executors/test_celery_executor.py:TestCeleryExecutor.test_error_sending_task
tests/jobs.py:SchedulerJobTest.test_change_state_for_tasks_failed_to_execute

Also updated existing failing unit test.

Before:
Before (32k tasks should run on 13:30 but didn't get >30k running tasks until 13:41):

After (32k tasks should run on 15:10 and got them all running at 15:14):

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff