Remove rz_th_kill, rz_th_kill_free and rz_th_pool_kill

[wargio]

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• I've added tests that prove my fix is effective or that my feature works (if possible)
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[thestr4ng3r]

RzThreadBool is not a very good name imo. What about calling it RzAtomicBool and adding a documentation comment that right now it's just super barebones and the "real" atomic ops like cas can be added later, as well as implementations without locks?