Replace time.time() with time.monotonic()

[max-rocket-internet]

Resolves #1487

I didn't replace it in parts related to stats as I'm not sure of the effect there. So I've replaced in these 3 parts where time is measured by subtracting a previous time from time.time().

FastHttpUser is not affected by this problem in my testing. It uses a timer from the timeit package.

[codecov]

Codecov Report

Merging #1492 into master will increase coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1492      +/-   ##
==========================================
+ Coverage   81.13%   81.51%   +0.37%     
==========================================
  Files          27       27              
  Lines        2386     2386              
  Branches      366      366              
==========================================
+ Hits         1936     1945       +9     
+ Misses        356      350       -6     
+ Partials       94       91       -3     

Impacted Files	Coverage Δ
locust/clients.py	95.09% <100.00%> (+4.90%)	⬆️
locust/user/task.py	96.21% <0.00%> (+0.54%)	⬆️
locust/runners.py	80.81% <0.00%> (+0.61%)	⬆️

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[cyberw]

I’m merging this because it is an improvement, but maybe we should look in to using timeit (like FastHttpUser is doing)