[AIRFLOW-1730] Unpickle value of XCom queried from DB

[mrkm4ntr]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1730

Description

• Here are some details about my PR, including screenshots of any UI changes:
  If enable_xcom_pickling is True, the value of XCom that queried from DB is not unpickled.
  The list of XComs not rendered correctly.

Tests

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

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
  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"

[codecov-io]

Codecov Report

Merging #2701 into master will increase coverage by 0.03%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #2701      +/-   ##
==========================================
+ Coverage    76.5%   76.54%   +0.03%     
==========================================
  Files         203      203              
  Lines       15025    15020       -5     
==========================================
+ Hits        11495    11497       +2     
+ Misses       3530     3523       -7

Impacted Files	Coverage Δ
airflow/models.py	87.31% <90.9%> (+0.26%)	⬆️

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[simonvanderveldt]

Any chance this can be merged for 1.9.1? The issue makes reading/checking our XCom values pretty problematic

[mrkm4ntr]

This PR has never been reviewed...
If it will be reviewed, I will resolve the conflict at any time.

[ashb]

This looks good (and the re-init trick is nice). Could you rebase/resolve and then I'll be able to merge.

Oh and mention me once done please!

[mrkm4ntr]

@ashb Thank you for your review! I resolved conflicts.