Smarter sync mechanism

[asonnino]

There are many ways to improve our current sync strategy.

For instance, if we get the same parent from more than one peer, we can ask these peers first before random selection.

An other example could be:

• Ask all nodes for missing data
• All nodes start streaming chunks (FC coded) of the data
• Stop streaming once we can reconstruct the data

All nodes keep in memory already FC-coded data in case others need to sync.