[AIRFLOW-6495] Load DAG only once when running a task using StandardTaskRunner

[mik-laj]

If the process is created using a fork, it will share all loaded modules that were available before forking. This allows us to avoid loading DAG again. Sometimes the DAG loading process is costly because it do database queries or other costly operation, so we should limit it if possible.

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Issue link: AIRFLOW-6495

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[codecov-io]

Codecov Report

Merging #7090 into master will decrease coverage by 0.28%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #7090      +/-   ##
==========================================
- Coverage   85.07%   84.79%   -0.29%     
==========================================
  Files         680      680              
  Lines       38810    38811       +1     
==========================================
- Hits        33019    32910     -109     
- Misses       5791     5901     +110

Impacted Files	Coverage Δ
airflow/task/task_runner/standard_task_runner.py	67.21% <50%> (+0.54%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.75% <0%> (-20%)	⬇️

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[turbaszek]

@mik-laj I'm afraid this won't work with --pickle flag:

airflow/airflow/cli/commands/task_command.py

Lines 211 to 212 in 10c026c

	if dag and args.pickle:
	raise AirflowException("You cannot use the --pickle option when using DAG.cli() method.")