It used to be declared as ulint, which is 32/64 bit integer depending on OS bitness.
However this declaration lead to problems with os_compare_and_swap_thread_id
implementation which operated on 32 bit integers (InterlockedCompareExchange)
Using 64 bit int lead to wrong result in os_compare_and_swap_thread_id
and to crash/assertion in rw_lock_set_writer_id_and_recursion_flag()
The fix is to use 4 byte integer to os_thread_id type definition.