replace element_neighbors_locks_ lockes with read locks where possible

[meiravgri]

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• [ ] This PR introduces API changes
• [ ] This PR introduces serialization changes

[codecov[bot]]

Codecov Report

Patch coverage: 96.22% and project coverage change: -0.12 :warning:

Comparison is base (fccd74b) 95.63% compared to head (f038427) 95.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
- Coverage   95.63%   95.52%   -0.12%     
==========================================
  Files          66       66              
  Lines        4173     4182       +9     
==========================================
+ Hits         3991     3995       +4     
- Misses        182      187       +5     

Impacted Files	Coverage Δ
src/VecSim/algorithms/hnsw/hnsw.h	98.35% <96.07%> (+0.01%)	:arrow_up:
src/VecSim/algorithms/hnsw/hnsw_batch_iterator.h	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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