refactor: 15946 Removed static fields from VirtualHasher and VirtualNodeCache

[imalygin]

Description:

This PR replaced VirtualNodeCache.cleaningPool and VirtualHasher.hashingPool with object-level fields.

Related issue(s):

Fixes #15946

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

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[lfdt-bot]

✅ Snyk checks have passed. No issues have been found so far.

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ 88.64%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (16910e1)	92861	76407	82.28%
Head commit (54e092f)	92854 (-7)	76404 (-3)	82.28% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#24061)	44	39	88.64%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

❌ Patch coverage is 88.63636% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...wirlds/virtualmap/internal/hash/VirtualHasher.java	69.23%	4 Missing ⚠️
...ds/virtualmap/internal/cache/VirtualNodeCache.java	95.83%	1 Missing ⚠️

@@            Coverage Diff            @@
##               main   #24061   +/-   ##
=========================================
  Coverage     78.28%   78.29%           
  Complexity    11656    11656           
=========================================
  Files          2461     2461           
  Lines         92958    92951    -7     
  Branches       9965     9962    -3     
=========================================
+ Hits          72772    72775    +3     
+ Misses        16503    16499    -4     
+ Partials       3683     3677    -6     

Files with missing lines	Coverage Δ	Complexity Δ
...c/main/java/com/swirlds/virtualmap/VirtualMap.java	76.18% <100.00%> (+0.04%)	0.00 <0.00> (ø)
...ds/virtualmap/internal/cache/VirtualNodeCache.java	77.72% <95.83%> (ø)	0.00 <0.00> (ø)
...wirlds/virtualmap/internal/hash/VirtualHasher.java	86.63% <69.23%> (-0.48%)	0.00 <0.00> (ø)

... and 12 files with indirect coverage changes

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