Restore stdout/stderr during shutdown.

[vamega]

This allows for programs that invoke locust programatically to continue to
operate in a non-surprising fashion after the locus call has completed.

[codecov-io]

Codecov Report

Merging #19 into programmatic will decrease coverage by 0.11%.
The diff coverage is 20%.

@@               Coverage Diff                @@
##           programmatic      #19      +/-   ##
================================================
- Coverage         65.96%   65.85%   -0.12%     
================================================
  Files                14       14              
  Lines              1472     1476       +4     
  Branches            234      234              
================================================
+ Hits                971      972       +1     
- Misses              434      437       +3     
  Partials             67       67

Impacted Files	Coverage Δ
locust/main.py	39.24% <0%> (-0.15%)	⬇️
locust/log.py	80% <33.33%> (-4.38%)	⬇️

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