[AIRFLOW-3462] Move TaskReschedule out of models.py

[seelmann]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-3462

Description

• Here are some details about my PR:
  • Class TaskReschedule is moved out of model.py/models/__init__.py
  • Some tweaks to avoid and eliminate circular dependencies (move ID_LEN to base, use strings instead of object references in foreign key constraint)

Tests

• My PR does not need testing for this extremely good reason:
  • Refactoring only

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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[feng-tao]

@seelmann , flake8 fails

[ashb]

Rebase to latest master to fix flake8

[codecov-io]

Codecov Report

Merging #4618 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4618      +/-   ##
==========================================
+ Coverage   74.26%   74.26%   +<.01%     
==========================================
  Files         423      424       +1     
  Lines       27788    27795       +7     
==========================================
+ Hits        20637    20643       +6     
- Misses       7151     7152       +1

Impacted Files	Coverage Δ
airflow/models/connection.py	64.24% <100%> (+0.21%)	⬆️
airflow/models/slamiss.py	93.33% <100%> (ø)	⬆️
airflow/jobs.py	77.38% <100%> (ø)	⬆️
airflow/models/taskreschedule.py	100% <100%> (ø)
airflow/models/__init__.py	92.44% <100%> (-0.14%)	⬇️
airflow/sensors/base_sensor_operator.py	97.91% <100%> (+0.04%)	⬆️
airflow/models/base.py	87.5% <100%> (+1.78%)	⬆️
airflow/ti_deps/deps/ready_to_reschedule.py	100% <100%> (ø)	⬆️

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[feng-tao]

LGTM