Improve block transaction download by using a better algorithm

[andreabadesso]

That's impossible by design. Blocks must have their parent block as first element in the parent list.

Hum, I'm still a little concerned because the DAG can get really deep sometimes. Maybe add a protection that will fail and report a problem or move to a DFS implementation using a stack (instead of a recursion).

Originally posted by @msbrogli in #1 (comment)

[andreabadesso]

Another potential solution for this issue is to use an API that returns all transactions that were confirmed by a given block