Allow Users to stop the runner by calling self.environment.runner.quit() (without deadlocking sometimes)

[cyberw]

No description provided.

[codecov]

Codecov Report

Merging #1448 into master will increase coverage by 0.16%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
+ Coverage   81.33%   81.49%   +0.16%     
==========================================
  Files          27       27              
  Lines        2357     2367      +10     
  Branches      362      366       +4     
==========================================
+ Hits         1917     1929      +12     
- Misses        349      351       +2     
+ Partials       91       87       -4     

Impacted Files	Coverage Δ
locust/runners.py	80.29% <0.00%> (+0.08%)	⬆️
locust/user/users.py	98.61% <100.00%> (+3.02%)	⬆️
locust/user/task.py	96.79% <0.00%> (+0.07%)	⬆️

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