MINOR: debug: add a trivial PRNG for scheduler stress-tests

MINOR: debug: add a trivial PRNG for scheduler stress-tests

Commit a5a447984 ("MINOR: debug: add "debug dev sched" to stress the
scheduler.") doesn't scale with threads because ha_random64() takes care
of being totally thread-safe for use with UUIDs. We don't need this for
the stress-testing functions, let's just implement a xorshift PRNG
instead. On 8 threads the performance jumped from 230k ctx/s with 96%
spent in ha_random64() to 14M ctx/s.