#1306 fix non-thread-safe usages of CopyOnWriteMap

[asolntsev]

Instead of two-three separate calls like containsKey, get and put, now we use a single computeIfAbsent.

P.S. I would like to also deny method CopyOnWriteMap.put(), but it's still used in one place (that was harder to refactor).

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 92.31%. Comparing base (a845854) to head (2b036b9).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
.../java/net/datafaker/service/FakeValuesService.java	92.36%	4 Missing and 6 partials ⚠️
.../net/datafaker/internal/helper/CopyOnWriteMap.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1381      +/-   ##
============================================
- Coverage     92.40%   92.31%   -0.10%     
- Complexity     3148     3150       +2     
============================================
  Files           319      319              
  Lines          6178     6144      -34     
  Branches        604      592      -12     
============================================
- Hits           5709     5672      -37     
- Misses          325      329       +4     
+ Partials        144      143       -1     

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[kingthorin]

This looks good to me, thanks for digging into the problem.