[fix] add checks for stopped queries

[serenajiang]

CATEGORY

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• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

When a user tries to stop a query, very occasionally, the action fails and the query still runs to completion. The issue isn't user-facing, but errors in the backend show that there is some kind of lock wait timeout issue.

Unknown if it is possible to completely prevent these, and the issue isn't really user-facing, but I added some tweaks to hopefully prevent a few of these conflicts.

• Add commit() after setting the initial query status to RUNNING
• Check if the query has stopped before running each statement. Exit if query has stopped.
• (Unrelated) Removed line 281 which (afaik) doesn't do anything

TEST PLAN

Tox passes.

I didn't write a test because it is pretty hard to reproduce this error - seems like a rare timing thing. These changes are mostly for safety and shouldn't affect behavior of anything.

REVIEWERS

@etr2460

[codecov-io]

Codecov Report

Merging #8097 into master will decrease coverage by 0.02%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #8097      +/-   ##
==========================================
- Coverage   65.83%   65.81%   -0.03%     
==========================================
  Files         486      486              
  Lines       22921    22924       +3     
  Branches     2521     2521              
==========================================
- Hits        15090    15087       -3     
- Misses       7700     7706       +6     
  Partials      131      131

Impacted Files	Coverage Δ
superset/sql_lab.py	76.84% <75%> (-0.17%)	⬇️
superset/db_engine_specs/postgres.py	79.31% <0%> (-17.25%)	⬇️

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[mistercrunch]

LGTM