maint: Replace `folly::ConcurrentHashMap` with `boost::concurrent_flat_map`

[jjerphan]

Reference Issues/PRs

To be merged after https://github.com/man-group/ArcticDB/pull/1576.

Towards #1412.

What does this implement or fix?

Making modification with the vcpkg-based workflow for now.

Any other comments?

Checklist

Checklist for code changes...

• [ ] Have you updated the relevant docstrings, documentation and copyright notice?
• [ ] Is this contribution tested against all ArcticDB's features?
• [ ] Do all exceptions introduced raise appropriate error messages?
• [ ] Are API changes highlighted in the PR description?
• [ ] Is the PR labelled as enhancement or bug so it appears in autogenerated release notes?

[jjerphan]

For conda-builds: we cannot use the boost>=1.84.0, libprotobuf<4, and pyarrow for now.

One of their stacks might need to be rebuilt so that they share common dependencies' versions.

[jjerphan]

Closing for now, might reopen one day.