Optimize block links

[Gilthoniel]

Currently, a link to a block is signed from the digest of the previous block. This is fine but not optimal because it's the block with the current roster that should sign it so that a chain to a block (the proof of consistency) is shaped with the smallest possible number of blocks. In other words, a proof to a block should look like this:

Genesis[R] --> Block[R'] --> Block[R''] --> Block[R'']

Basically, every block that has a change in the roster is a new checkpoint for the chain. New blocks should always be signed with the most recent roster to prevent a group of malicious nodes to take over the chain (i.e. an old roster with few nodes thus a small failure threshold).