tree/ev_listener_actor: destroy callback as early as possible
tree/ev_listener_actor: destroy callback as early as possible

By explicitly destructing callback in `on_exit()` we get:
1. Deterministic behavior of predictable state destruction.
2. All Python callbacks are released after behavior stop for sure.

This fixes accidents when Python callbacks being destructed while ev
listener actors cleanup AFTER Python is already dead.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re