core: allow for 0ns simulation steps and synchronization points

[MariusBgm]

PoC: trigger a simulation step with 0ns duration, for certain message types (which implement the SIlKitMsgTrait<T>::IsSynchronizationPoint) TODO:

• need to guard the behavior with handshake capabilities? -> don't send zero timestamps to participants which do not understand them
• we should have an explicit synchronization message, independent of the NextSimTask. E.g., for flow control and backpressure in non-virtual-time participants TODO: look at barriers, e.g. the RequestReplyService.