Write up Crosslink 2

[daira]

Add this to Potential Changes to Crosslink.

Make the book describe Crosslink 2 as the baseline (and motivate Crosslink 2*). Crosslink 1 can be relegated to a "roads not taken" page.

Basically the idea in Crosslink 2 is to replace the Increasing Score rule in Crosslink with:

Linearity rule: $\mathsf{snapshot}(B \lceil_{\mathrm{bft}}^1) \preceq_{\mathrm{bc}} \mathsf{snapshot}(B)$.

This is at least as safe because the Linearity rule implies the Increasing Score rule. It has many consquences!

• Sanitization is not required.
• The rule for honest bc-block production needs to be modified to be consistent with this rule.
• The liveness analysis has to be redone.

Crosslink 2* additionally unifies bft-block-producers with miners.