ITS: allow sharing of arena in Tracker & Vertexer

[f3sch]

This lets us get rid of the code duplication due to the double loops but also restores fully the single threaded performance. Thus I decided to leave single threaded the default.

I would prefer not to squash this.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 92d5d19 at 2025-06-16 07:18:

input queue has size 553
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input queue has size 0
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++ cp thinned_compile_commands.json compile_commands.json
++ CHECKS='-*,modernize-avoid-bind,modernize-deprecated-headers,modernize-make-shared,modernize-raw-string-literal,modernize-redundant-void-arg,modernize-replace-auto-ptr,modernize-replace-random-shuffle,modernize-shrink-to-fit,modernize-unary-static-assert,modernize-use-equals-default,modernize-use-noexcept,modernize-use-nullptr,modernize-use-override,modernize-use-transparent-functors,modernize-use-uncaught-exceptions,readability-braces-around-statements'
++ tee error-log.txt
+++ which O2codecheck
+++ find /sw/slc9_x86-64/GCC-Toolchain/v14.2.0-alice2-1/lib -name crtbegin.o -exec dirname '{}' ';'
++ run_O2CodeChecker.py -j 32 -clang-tidy-binary /sw/slc9_x86-64/o2codechecker/v18.1.2.1-local1/bin/O2codecheck -clang-apply-replacements-binary /sw/slc9_x86-64/Clang/v18.1.8-21/bin-safe/clang-apply-replacements -extra-args=--extra-arg=--gcc-install-dir=/sw/slc9_x86-64/GCC-Toolchain/v14.2.0-alice2-1/lib/gcc/x86_64-unknown-linux-gnu/14.2.0 '-header-filter=.*SOURCES(?!.*/3rdparty/).*' '-checks=-*,modernize-avoid-bind,modernize-deprecated-headers,modernize-make-shared,modernize-raw-string-literal,modernize-redundant-void-arg,modernize-replace-auto-ptr,modernize-replace-random-shuffle,modernize-shrink-to-fit,modernize-unary-static-assert,modernize-use-equals-default,modernize-use-noexcept,modernize-use-nullptr,modernize-use-override,modernize-use-transparent-functors,modernize-use-uncaught-exceptions,readability-braces-around-statements'
No checks enabled.
Unable to run clang-tidy.

Full log here.

[f3sch]

@mconcas @fprino only when you have time could you take a look?