Use proper startup notification for backend

[maringuu]

In our testing codebase we have several calls to sleep to wait that "everything is ready".

This is not very reliable and bad practice.

My proposal is to use signals to signal readyness to the parent process.

[jstucke]

I think that is a good idea. I already replaced some sleep()s in the past with Events which are ideal for this kind of thing. See e.g. src/test/integration/scheduler/test_unpack_analyse_and_compare.py