Lifetime events for participant

[bergie]

Right now participants can only react to incoming packets. This means a participant that starts sending immediately can't be done in msgflo-python

Needed for https://github.com/c-base/farbgeber

[jonnor]

With #20 one can also an IIP will be delivered on startup, so this case can be handled that way