Rename "deadlock" to "stall" in LoadManager

[Bronek]

High Level Overview of Change

In the condition of severe CPU starvation, LoadManager will intentionally crash rippled reporting LogicError: Deadlock detected. This error message is misleading; the condition being detected is not a deadlock. Let's change the name accordingly.

Context of Change

In the context of multithreaded programs, 'deadlock' has a specific meaning and what the LoadManager class does, has nothing to do with it. At the time when this class was originally written, std::mutex etc. primitives were unable to report actual deadlocks, so this class LoadManager was used instead to detect a lack of progress in JobQueue.

Since then we have better tools, and the only thing this class is left doing is stall detection, which is not the same as deadlock detection.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

[codecov]

Codecov Report

Attention: Patch coverage is 28.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 78.1%. Comparing base (2216e5a) to head (0059871).

Files with missing lines	Patch %	Lines
src/xrpld/app/main/LoadManager.cpp	30.4%	16 Missing ⚠️
src/xrpld/app/main/Application.cpp	0.0%	1 Missing ⚠️
src/xrpld/app/misc/NetworkOPs.cpp	0.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5341     +/-   ##
=========================================
- Coverage     78.1%   78.1%   -0.0%     
=========================================
  Files          790     790             
  Lines        67908   67910      +2     
  Branches      8230    8229      -1     
=========================================
- Hits         53034   53025      -9     
- Misses       14874   14885     +11     

Files with missing lines	Coverage Δ
src/xrpld/app/main/Application.cpp	69.3% <0.0%> (ø)
src/xrpld/app/misc/NetworkOPs.cpp	69.1% <0.0%> (ø)
src/xrpld/app/main/LoadManager.cpp	56.4% <30.4%> (-0.2%)	⬇️

... and 3 files with indirect coverage changes

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