perf(caching): improves caching of loader and optimizer

[OlenDavis]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

1. Performance refactor / race-condition fix:

This PR eliminates the race condition where the loader or optimizer may do the same processing (same transformation on the same image) redundantly if the same file transformation's loader is triggered more than once before the first transformation has finished.

2. Fix npm start

Didn't work before; now it does. (With as little change to the prior scripts as possible.)

Breaking Changes

None that I'm aware of.

Additional Info

This PR doesn't eliminate all possible such race conditions, namely if the same image transformation occurs between the loader and the minimizer, because the transformers are different functions (because the minimizer functionality is different than the generator/loader, even though the underlying operation is the same), it's still possible to have redundant image transformations if one transformation happens via the loader, and the same transformation happens via the webpack optimize hook.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.15%. Comparing base (3a924e4) to head (542ad09).
Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
src/loader.js	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
+ Coverage   77.16%   78.15%   +0.98%     
==========================================
  Files           4        4              
  Lines         727      769      +42     
  Branches      282      304      +22     
==========================================
+ Hits          561      601      +40     
- Misses        137      139       +2     
  Partials       29       29              

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