query: Use random retry in busy handler
query: Use random retry in busy handler

- Have the busy handler choose the backoff delay randomly (but never more
than 5 ms), rather than choosing it deterministically based on the number
of attempts

- Add backoff_delay() helper function

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr