We spend additional size_of(std::thread::id) bytes to gather sum (or xor) of reader thread IDs. In case there is only one, we will have the reliable thread ID value retrieved to present to user. Note this will not be the last thread that took S-latch, but the one that really is still holding the S-latch.
os_thread_id and other members of thread0os.h was removed and replaced with standard's std::thread and std::this_thread infrastructure.
Added a unit test with a general execute_multithreaded_phase_plan() allowing to prepare and execute multi-threaded test plan.

RB#25661