detect scheduling stalls event-wise, and don't force Avalanche Scheduler to self-deactivate upon a failure/stall (the latter is still handled by HiveSlimEventLoopMgr)

See merge request !667

Closes #28

Old definition was using the class keyword, the new one was initially using struct and so any forward declaration created a clang warning.
We are now back to class.

add support for multiple CF parents to topological execution flow tracer, and verbose/error messages cleanup in PRG