[MAJOR] use an ebtree instead of a list for the run queue

[MAJOR] use an ebtree instead of a list for the run queue

We now insert tasks in a certain sequence in the run queue.
The sorting key currently is the arrival order. It will now
be possible to apply a "nice" value to any task so that it
goes forwards or backwards in the run queue.

The calls to wake_expired_tasks() and maintain_proxies()
have been moved to the main run_poll_loop(), because they
had nothing to do in process_runnable_tasks().

The task_wakeup() function is not inlined anymore, as it was
only used at one place.

The qlist member of the task structure has been removed now.
The run_queue list has been replaced for an integer indicating
the number of tasks in the run queue.