Fixes #20. Not as easy as i thought it was.
The old backchannel implementation sent same data to "ALL" flakes for "ALL" predecessor pellets.
However, since we support more than one incoing edges with different channel types. That wont work.
Changed the backchannel implementation to support multiple edge types and multiple incoming edges.

Also added the interface to get the backchannel data from the FlakeLocalStrategy since back channel and this strategy are tightly coupled.