Add QueryableJobExecutionStorageInterface::purge

[yann-eugone]

See #97

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (98ae357) to head (b4b6fcc).
⚠️ Report is 1 commits behind head on 1.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                1.x     #187   +/-   ##
=========================================
  Coverage     99.32%   99.32%           
- Complexity      997     1000    +3     
=========================================
  Files           172      172           
  Lines          3119     3127    +8     
=========================================
+ Hits           3098     3106    +8     
  Misses           21       21           

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