Add finality certificate capture to Observer

[masih]

The work in #744 introduced a new observer implementation with SQL query over all captured GPBFT messages. But we want more:

• ratio of participating power in the network for any given instance
• finality certificate content for debugging purposes

The former can be extracted using cert exchange protocol, similar to the approach implemented here. Having this info is super useful because it means we would no longer require running a lotus node to get the power participation information. It would also mean we can use a single SQL interface for debugging which is super expressive.

Other future benefits include, a rolling snapshot of all finality certificates generated by F3 in case we need it for disaster recovery.

[rjan90]

Does not block a release, but for our infra.