Implement Consensus Chain Handling Services

[teor2345]

Here's a rough sketch of the different services we need to validate and maintain the main chain. See #682 for the full chain state design.

• ChainVerifier (Implement the chain verifier service #691)

  • CheckpointVerifier (Block checkpointing service. #429): Rapidly syncs older blocks using a checkpoint list.
  • BlockVerifier (Tracking: block validation #428): Verifies newer blocks and their transactions, using consensus rules.
  • state service: adds verified blocks to the state

• ChainTipService (Handle chain reorganisations #452, Chain State Design #682)

  • Chooses the chain tip with the greatest cumulative proof of work.
  • Updates chain state when the chain tip changes. Handles chain extensions and reorganisations.
    • add block
    • update block height map
    • (and other state as needed)
  • Deletes side-chains that are a long way behind the chosen chain tip.

We might want to abstract over verification and state updates, but let's decide that after we've implemented stubs for each service.

[teor2345]

Obsoleted by #763 and other RFCs.