- uasync_set_timeout(0): use FIFO immediate_queue instead of timeout_heap
to guarantee execution AFTER all currently-expired heap timers
- uasync_cancel_timeout: search immediate_queue for timeout=0 timers
- dummynet: start initial shaper with bandwidth-based delay instead of 0
(prevents queue from never filling up when packets arrive sequentially)
- test_dummynet: rename get_time_us -> now_us (conflict with u_async.h)
- test_u_async_comprehensive: fix immediate_timeouts, add timeout_ordering test
Evgeny