Implement a worker thread for condition refresh

[mregen]

Proposed changes

Currently ConditionRefresh requests are queued and executed on tasks. This change adds requests to a queue and refresh is executed by a worker, limiting the amount of threads that can execute concurrently.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

[codecov]

Codecov Report

Merging #2105 (434352d) into master (bd0f02a) will increase coverage by 0.19%.
The diff coverage is 64.51%.

@@            Coverage Diff             @@
##           master    #2105      +/-   ##
==========================================
+ Coverage   57.88%   58.08%   +0.19%     
==========================================
  Files         324      324              
  Lines       61606    61649      +43     
==========================================
+ Hits        35663    35807     +144     
+ Misses      25943    25842     -101     

Impacted Files	Coverage Δ
.../Opc.Ua.Server/Subscription/SubscriptionManager.cs	70.87% <63.33%> (+1.03%)	⬆️
...aries/Opc.Ua.Server/NodeManager/ResourceManager.cs	65.76% <100.00%> (-0.18%)	⬇️

... and 19 files with indirect coverage changes

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[mrsuciu]

👍