Fix MaxClientsAllowed

[amoerie]

Fixes #1669 .

Checklist

• The pull request branch is in sync with latest commit on the fo-dicom/development branch
• I have updated API documentation
• I have included unit tests
• I have updated the change log
• I am listed in the CONTRIBUTORS file

Changes proposed in this pull request:

• Use Semaphore.WaitAsync with timeout to avoid entering the Semaphore and causing MaxClientsAllowed to get stuck

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f6564f5) 76.40% compared to head (882d08a) 76.39%.

❗ Current head 882d08a differs from pull request most recent head fa89686. Consider uploading reports for the commit fa89686 to get more accurate results

Files	Patch %	Lines
FO-DICOM.Core/Network/DicomServer.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development    #1670      +/-   ##
===============================================
- Coverage        76.40%   76.39%   -0.02%     
===============================================
  Files              274      274              
  Lines            25168    25153      -15     
  Branches          3008     3001       -7     
===============================================
- Hits             19230    19216      -14     
+ Misses            5017     5015       -2     
- Partials           921      922       +1     

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