Try to keep only a single Config instance in memory

[z4kn4fein]

Describe the purpose of your pull request

Trying to keep only a single Config instance in memory at once.

Requirement checklist (only if applicable)

• [x] I have covered the applied changes with automated tests.
• [x] I have executed the full automated test set against my changes.
• [x] I have validated my changes against all supported platform versions.
• [x] I have read and accepted the contribution agreement.

[codecov[bot]]

Codecov Report

Merging #11 (b70e7d3) into master (fc4aedb) will decrease coverage by 0.07%. The diff coverage is 90.00%.

:exclamation: Current head b70e7d3 differs from pull request most recent head df5eb7c. Consider uploading reports for the commit df5eb7c to get more accurate results

@@             Coverage Diff              @@
##             master      #11      +/-   ##
============================================
- Coverage     80.31%   80.23%   -0.08%     
  Complexity      285      285              
============================================
  Files            26       26              
  Lines           828      835       +7     
  Branches        113      107       -6     
============================================
+ Hits            665      670       +5     
- Misses          117      122       +5     
+ Partials         46       43       -3     

Impacted Files	Coverage Δ
src/main/java/com/configcat/AutoPollingMode.java	100.00% <ø> (ø)
src/main/java/com/configcat/PollingModes.java	75.00% <ø> (ø)
src/main/java/com/configcat/FetchResponse.java	93.33% <66.66%> (-6.67%)	:arrow_down:
src/main/java/com/configcat/ConfigJsonCache.java	85.71% <85.71%> (ø)
src/main/java/com/configcat/LazyLoadingPolicy.java	85.29% <87.50%> (-0.82%)	:arrow_down:
src/main/java/com/configcat/ConfigFetcher.java	83.69% <90.90%> (+3.47%)	:arrow_up:
src/main/java/com/configcat/AutoPollingPolicy.java	94.28% <100.00%> (-0.31%)	:arrow_down:
src/main/java/com/configcat/ConfigCatClient.java	56.87% <100.00%> (-1.15%)	:arrow_down:
src/main/java/com/configcat/ConfigModels.java	100.00% <100.00%> (+23.52%)	:arrow_up:
...c/main/java/com/configcat/ManualPollingPolicy.java	100.00% <100.00%> (ø)
... and 4 more

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[sonarcloud[bot]]

Please retry analysis of this Pull-Request directly on SonarCloud.

[kp-cat]

Maybe ConfigCatClient.getKeyAndValueFromConfig() should be renamed to ConfigCatClient.getKeyAndValueFromSettingsMap() for consisntecy.