Bug: new node can't download blocks after starting from a snapshot

[iltoga]

Description

Because of how the block validation works, new nodes that starts downloading a snapshot get stuck at some point and can't finish downloading the rest of main blocks.
As a solution we are going to include in a snapshot the 720 blocks (minRollBackHeight) previous to snapshotHeight.

Steps to reproduce

1. run a node till generates a snapshot and let it smith some 10's more main blocks
2. run a second node
3. verify that the second node is not able to download all remaining blocks from first node

Expected behavior

A node should always be able to finish downloading main blocks