Add support for lock-free and deterministic use of Random_XorShift*_Pool
by giving state_idx selection and lock responsibility up to the
application. Done by an overload of get_state() to take sate_idx as
an argument that the appplication guarantees is concurrently unique
and within the range of num_states that the application passed to init().
In other words, this allows the RNG state to be associated with some
application specific index, rather than a runtime arbitrary thread ID,
and thus the application can control which work is performed using
which RNG in a deterministic manner, regardless of which thread
performs the work.
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