BUG/MINOR: haload: fix spurious task wakeup in hld_strm_task()
When a stream is freed, hld_strm_task() was unconditionally re-queuing
the user task, even when no streams were left or when no update was
needed. This caused spurious task wakeups, which could incorrectly inflate
connection and request rate counters to come for the -R option implementation.
Only queue the user task if there are remaining streams to process, and
properly update the expiration time.
No need to backport. haload arrived with this current version.