Parallelize host-side hashing and client-side hashing.

[BrianPugh]

@raveslave see if this speeds anything up for you.

[codecov]

Codecov Report

Attention: Patch coverage is 53.12500% with 15 lines in your changes missing coverage. Please review.

Project coverage is 70.66%. Comparing base (5e68403) to head (8400dfa).

Files with missing lines	Patch %	Lines
belay/device.py	53.12%	7 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
+ Coverage   70.65%   70.66%   +0.01%     
==========================================
  Files          39       39              
  Lines        1707     1708       +1     
  Branches      382      384       +2     
==========================================
+ Hits         1206     1207       +1     
  Misses        413      413              
  Partials       88       88              

Flag	Coverage Δ
unittests	70.43% <53.12%> (+0.01%)	⬆️

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