Fast "header-first" block sync

[sdbondi]

Tracking issue for fast block sync.

Fast block sync will attempt to establish the strongest PoW chain by checking where a local chain and a remote peer's chain are linked and downloading headers from that point to the tip. Once headers are validated and downloaded, an archival node proceeds to block sync where full block bodies are downloaded.

• Base node sync RPC service (PR Base node block sync RPC service #2348)
• Various small bug fixes / enhancements in base node sync RPC
• Database backend support for partial state
  • Support rewinding with or without bodies
  • Support adding/associating block bodies with existing headers
• Change state machine to complete header sync before block sync
• Validate headers efficiently by building and maintaining an in-memory cache of target difficulties and timestamps for the block window
• Validate block bodies against header MMRs
• Chain metadata must adjust as necessary to reflect which full blocks the node can provide
• Sync peers should be selected at random for header sync
• Allow "force sync from this peer" to be configured
• Block sync needs to locate a peer that can provide the required full blocks

Related issues

#2113