Fix subtle memory leak issue

[adams85]

Describe the purpose of your pull request

A hook event handler which closes over the client instance should not prevent the client from being collected by the GC when user has no more references to the client.

Requirement checklist (only if applicable)

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

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (2b8f955) 91.75% compared to head (1fe7fa5) 91.65%.

❗ Current head 1fe7fa5 differs from pull request most recent head 5b0c149. Consider uploading reports for the commit 5b0c149 to get more accurate results

Files	Patch %	Lines
src/ConfigCatClient.ts	64.70%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   91.75%   91.65%   -0.11%     
==========================================
  Files          18       18              
  Lines        1613     1629      +16     
  Branches      408      411       +3     
==========================================
+ Hits         1480     1493      +13     
- Misses        132      135       +3     
  Partials        1        1              

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication