Resolve deadlock between harvest thread and JVM shutdown thread

[jtduffy]

Resolves #2514

Replaces the synchronized methods and code blocks with a ReentrantLock instance. In the harvestNow() method, the tryLock() method is called with a 200ms timeout, so even if two threads are fighting for the lock, they will never be deadlocked since the tryLock will timeout and continue.

Wrote a few unit tests to (hopefully) reproduce the scenario and verify no deadlocks occur.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.50000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.30%. Comparing base (fe0fa9e) to head (a319b01).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
...t/src/main/java/com/newrelic/agent/RPMService.java	67.50%	8 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2539      +/-   ##
============================================
- Coverage     70.32%   70.30%   -0.02%     
- Complexity    10059    10117      +58     
============================================
  Files           847      855       +8     
  Lines         40913    41109     +196     
  Branches       6201     6232      +31     
============================================
+ Hits          28771    28901     +130     
- Misses         9343     9385      +42     
- Partials       2799     2823      +24     

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