MEDIUM: spoe: Use an ebtree to manage idle applets

MEDIUM: spoe: Use an ebtree to manage idle applets

Instead of using a list of applets with idle ones in front, we now use an
ebtree. Aapplets in the tree are idle by definition. And the key is the applet's
weight. When a new frame is queued, the first idle applet (with the lowest
weight) is woken up and its weight is increased by one. And when an applet sends
a frame to a SPOA, its weight is decremented by one.

This is empirical, but it should avoid to overuse a very few number of applets
and increase the balancing between idle applets.