timers-urgent-2020-07-19

 - A timer which is already expired at enqueue time can set the
   base->next_expiry value backwards. As a consequence base->clk can be set
   back as well. This can lead to timers expiring early. Add a sanity check
   to prevent this.

 - When a timer is queued with an expiry time beyond the wheel capacity
   then it should be queued in the bucket of the last wheel level which is
   expiring last. The code adjusts expiry time to the maximum wheel
   capacity, which is only correct when the wheel clock is 0. Aside of that
   the check whether the delta is larger than wheel capacity does not check
   the delta, it checks the expiry value itself. As a result timers can
   expire at random.

   Fix this by checking the right variable and adjust expiry time so it
   becomes base->clock plus capacity which places it into the outmost
   bucket in the last wheel level.