fix: add protections against dataraces in socket engines, cluster timer and queues.

[fclivaz42]

As discussed, here is a smaller PR with less risky changes.
I have changed the following:

• epoll, where I removed an unused variable and added a lock where elements were removed from the fds vector
• kqueue, where I have added a lock similar to epoll
• queues, where I have changed two booleans to std::atomic booleans, added a safeguard for the cli variable and guards for the removals vector.
• timer, where I added a guard to an unguarded if statement.

I believe these changes are light enough to not cause any issues whatsoever, and have kept the riskier changes out of this PR for now while I do more testing on my end.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[egorpugin]

My simple echo bots are stuck in epoll even with 10.1.5 version.
Does this fixes exactly those issues?

[fclivaz42]

My simple echo bots are stuck in epoll even with 10.1.5 version. Does this fixes exactly those issues?

You could try to test run by applying this PR as a patch and compile/install the library then test it. Feedback would actually be very welcome!

[braindigitalis]

are you saying a simple test bot now gets stuck in epoll and didn't before 10.1.5?