FIX Make threaded backend multiprocess-safe

[eigenein]

IF we configure ThreadedBackend and use it for both an app and Celery worker, then metrics are not being sent from Celery tasks because queue.Queue doesn't work between processes.

Options considered:

• Testing if we're in a worker and using a different backend. Looks quite dirty since Celery does not provide a built-in way to test this.
• Making ThreadedBackend work in multi-process environment.

[codecov-io]

Codecov Report

Merging #33 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #33   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      6    -1     
  Lines         180    180           
=====================================
  Hits          180    180

Impacted Files	Coverage Δ
time_execution/backends/threaded.py	100% <100%> (ø)	:arrow_up:
time_execution/backends/__init__.py

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[jeduden]

What issue is fixed by this PR ? I.e why are two separate processes interferring with each other?

[eigenein]

@jeduden Updated the description.