Stop locking on every handler call

[moggers87]

Salmon should have been thread-safe and performant from the start. This PR makes that true.

• Write tests for locking handler call
• Write tests for locking and nolocking decorators
• Make sure state storages are thread-safe (I think they are already, but it's worth checking)
• Update docs

[codecov]

Codecov Report

Merging #153 (f460707) into 🐟 (3894055) will increase coverage by 0.02%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##                🐟     #153      +/-   ##
==========================================
+ Coverage   93.98%   94.00%   +0.02%     
==========================================
  Files          15       15              
  Lines        1513     1519       +6     
  Branches      266      266              
==========================================
+ Hits         1422     1428       +6     
  Misses         65       65              
  Partials       26       26              

Impacted Files	Coverage Δ
salmon/server.py	90.85% <ø> (ø)
salmon/testing.py	72.41% <50.00%> (-0.81%)	⬇️
salmon/handlers/queue.py	66.66% <100.00%> (-3.34%)	⬇️
salmon/routing.py	96.99% <100.00%> (+0.50%)	⬆️

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