fix: 15438: Eliminate busy loop in HalfDiskHashMap.endWriting()

[artemananiev]

Fix summary:

• HDHM.endWriting() is reimplemented using fork-join pool and tasks
• No new unit tests are introduced, since endWriting() is well covered by existing tests

Fixes: #15438
Signed-off-by: Artem Ananev artem.ananev@swirldslabs.com

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.02% (target: -1.00%)	✅ 87.07%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7a4616b)	108968	67303	61.76%
Head commit (9aa566d)	109124 (+156)	67426 (+123)	61.79% (+0.02%)

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 (#15439)	116	101	87.07%

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

Attention: Patch coverage is 82.75862% with 20 lines in your changes missing coverage. Please review.

Project coverage is 58.25%. Comparing base (7a4616b) to head (9aa566d).
Report is 14 commits behind head on develop.

Files with missing lines	Patch %	Lines
...wirlds/merkledb/files/hashmap/HalfDiskHashMap.java	82.75%	15 Missing and 5 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #15439      +/-   ##
=============================================
+ Coverage      58.22%   58.25%   +0.02%     
- Complexity     21573    21599      +26     
=============================================
  Files           2779     2781       +2     
  Lines         109151   109325     +174     
  Branches       11163    11181      +18     
=============================================
+ Hits           63554    63686     +132     
- Misses         41738    41774      +36     
- Partials        3859     3865       +6     

Files with missing lines	Coverage Δ
...va/com/swirlds/merkledb/config/MerkleDbConfig.java	92.30% <ø> (ø)
...wirlds/merkledb/files/hashmap/HalfDiskHashMap.java	74.70% <82.75%> (-0.94%)	⬇️

... and 35 files with indirect coverage changes

[OlegMazurov]

Pending performance testing.