wip sync rework

[Mirko-von-Leipzig]

Looking for feedback on this wip sync redesign. Specifically, the async fn track() part (the rest is just me trying to think atm).

I'm trying to use futures instead of message / events.

[sergey-melnychuk]

Interesting initiative! For me personally it is too early to give any meaningful feedback. I'd try to add some tests or even proceed in a TDD manner. When things get too tricky to test in a straightforward way, it is a sign of a necessary change (likely a new abstraction to be added). Once a working prototype (at leas basic sync is handled: new block is processed, no reorg, no "healing" etc) is ready (with meaningful test coverage), then providing feedback is going to be much easier, at least for me.