[AIRFLOW-910] Use parallel task execution for backfills

[bolkedebruin]

The refactor to use dag runs in backfills caused a regression
in task execution performance as dag runs were executed
sequentially. Next to that, the backfills were non deterministic
due to the random execution of tasks, causing root tasks
being added to the non ready list too soon.

This updates the backfill logic as follows:
* Parallelize execution of tasks
* Use a leave first execution model; Breadth-first algorithm by Jerermiah
* Replace state updates from the executor by task based only

Please accept this PR that addresses the following issues:

• (MANDATORY - replace with a link to JIRA - e.g. https://issues.apache.org/jira/browse/AIRFLOW-XXX)

Testing Done:

• Unittests are required, if you do not include new unit tests please
  specify why you think this is not required. We like to improve our
  coverage so a non existing test is even a better reason to include one.

Reminders for contributors (REQUIRED!):

• Your PR's title must reference an issue on
  Airflow's JIRA.
  For example, a PR called "[AIRFLOW-1] My Amazing PR" would close JIRA
  issue Improving the search functionality in the graph view #1. Please open a new issue if required!

• For all PRs with UI changes, you must provide screenshots. If the UI changes are not obvious, either annotate the images or provide before/after screenshots.

• Please squash your commits when possible and follow the How to write a good git commit message.
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  1. Separate subject from body with a blank line
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  5. Wrap the body at 72 characters
  6. Use the body to explain what and why vs. how

[mention-bot]

@bolkedebruin, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mistercrunch, @jlowin and @plypaul to be potential reviewers.

[codecov-io]

Codecov Report

Merging #2145 into master will increase coverage by 0.01%.
The diff coverage is 83.59%.

@@            Coverage Diff             @@
##           master    #2145      +/-   ##
==========================================
+ Coverage   67.17%   67.19%   +0.01%     
==========================================
  Files         142      142              
  Lines       10769    10769              
==========================================
+ Hits         7234     7236       +2     
+ Misses       3535     3533       -2

Impacted Files	Coverage Δ
airflow/jobs.py	73.36% <81.98%> (ø)	✅
airflow/models.py	86.86% <94.11%> (+0.05%)	✅
airflow/executors/dask_executor.py	81.39% <0%> (+2.32%)	✅

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