fix: profiler reactor initialization

[glevco]

Motivation

The get_cpu_profiler() function which is called at the module-level in multiple places indirectly installed the Twisted reactor by calling LoopingCall(). This meant that when a module with a profiler was imported before reactor initialization (in initialize_global_reactor), a ReactorAlreadyInstalledError could be thrown.

This PR prevents the profiler from indirectly installing a reactor before it has been initialized.

Acceptance Criteria

• Change SimpleCPUProfiler so it does not create a LoopingCall in its __init__.
• Remove some unused calls to get_cpu_profiler().

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.83%. Comparing base (14289d5) to head (8a733dd).

Files	Patch %	Lines
hathor/profiler/cpu.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1058      +/-   ##
==========================================
- Coverage   84.92%   84.83%   -0.10%     
==========================================
  Files         300      300              
  Lines       22944    22936       -8     
  Branches     3471     3471              
==========================================
- Hits        19486    19458      -28     
- Misses       2769     2784      +15     
- Partials      689      694       +5     

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