ChainDB chain selection: account for weights/boosts by certificates

[amesgen]

Currently, chain selection is written under the assumption that chains can be compared purely based on their tip header (specifically, their SelectView). With Peras, this has to change in order to account for certificates giving a boost/extra weight to certain blocks, which can dynamically change over time.

The goal of this ticket is to:

1. Design a mechanism for the abstract Consensus layer how chains can be compared while taking weight/boosts by certificates into account.

2. Explain how to modify the existing ChainSel logic to account for certificates. In particular, certificates will usually arrive later than the block they are providing a boost for.