Reduce "start-up" time for tasks in LocalExecutor

[ashb]

Spawning a whole new python process and then re-loading all of Airflow
is expensive. All though this time fades to insignificance for long
running tasks, this delay gives a "bad" experience for new users when
they are just trying out Airflow for the first time.

For the LocalExecutor this cuts the "queued time" down from 1.5s to 0.1s
on average.

Data on this for a simple 10-task sequential DAG:

SELECT execution_date,
    min(start_date - queued_dttm) AS min_quued_delay,
    max(start_date - queued_dttm) AS max_queued_delay
FROM task_instance
WHERE dag_id = 'scenario1_case2_03_1' GROUP BY execution_date;

execution_date	min_quued_delay	max_queued_delay	with change?
2020-10-07 02:00:00+01	00:00:01.329747	00:00:01.475085	No
2020-10-07 01:00:00+01	00:00:00.083668	00:00:00.11244	Yes

This was discovered in my general benchmarking and profiling of the scheduler for AIP-15, but it's not tied to any of that work. There are more of these kind of improvements coming, each unrelated but all add up.

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[ashb]

[ashb]

I was thinking I'd follow up with a separate PR to add the same to CeleryExecutor.

[ashb]

I just noticed that we still need a fork in here - task_run in cli.commands does a logging.shutdown (and we need that as it does the remote upload) so I'll change this to fork first.

[ashb]

Updated benchmarks -- the fork is basically free.

[ashb]

Hmmm, something broken in all the kube test unrelated to this.

./scripts/ci/kubernetes/ci_run_kubernetes_tests.sh: /home/runner/work/airflow/airflow/.build/.kubernetes_venv/bin/pip: /home/runner/work/airflow/airflow/.build/.kubernetes_venv/bin/python: bad interpreter: No such file or directory

[ashb]

Oh, those kube tests will fail because I haven't re-triggered the image build. If the rests of the tests pass I'll merge this.