Implement checkpoint sync (#2244)

## Issue Addressed

Closes #1891
Closes #1784

## Proposed Changes

Implement checkpoint sync for Lighthouse, enabling it to start from a weak subjectivity checkpoint.

## Additional Info

- [x] Return unavailable status for out-of-range blocks requested by peers (#2561)
- [x] Implement sync daemon for fetching historical blocks (#2561)
- [x] Verify chain hashes (either in `historical_blocks.rs` or the calling module)
- [x] Consistency check for initial block + state
- [x] Fetch the initial state and block from a beacon node HTTP endpoint
- [x] Don't crash fetching beacon states by slot from the API
- [x] Background service for state reconstruction, triggered by CLI flag or API call.

Considered out of scope for this PR:

- Drop the requirement to provide the `--checkpoint-block` (this would require some pretty heavy refactoring of block verification)


Co-authored-by: Diva M <divma@protonmail.com>

beacon_node/network/src/service.rs

@@ -63,10 +63,8 @@ pub enum NetworkMessage<T: EthSpec> {
         response: Response<T>,
         id: PeerRequestId,
     },
-    /// Respond to a peer's request with an error.
-    SendError {
-        // NOTE: Currently this is never used, we just say goodbye without nicely closing the
-        // stream assigned to the request
+    /// Sends an error response to an RPC request.
+    SendErrorResponse {
         peer_id: PeerId,
         error: RPCResponseErrorCode,
         reason: String,
@@ -386,7 +384,7 @@ fn spawn_service<T: BeaconChainTypes>(
                         NetworkMessage::SendResponse{ peer_id, response, id } => {
                             service.libp2p.send_response(peer_id, id, response);
                         }
-                        NetworkMessage::SendError{ peer_id, error, id, reason } => {
+                        NetworkMessage::SendErrorResponse{ peer_id, error, id, reason } => {
                             service.libp2p.respond_with_error(peer_id, id, error, reason);
                         }
                         NetworkMessage::UPnPMappingEstablished { tcp_socket, udp_socket} => {