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/eth2_libp2p/src/peer_manager/mod.rs

@@ -578,8 +578,17 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
                 RPCResponseErrorCode::Unknown => PeerAction::HighToleranceError,
                 RPCResponseErrorCode::ResourceUnavailable => {
                     // NOTE: This error only makes sense for the `BlocksByRange` and `BlocksByRoot`
-                    // protocols. For the time being, there is no reason why a peer should send
-                    // this error.
+                    // protocols.
+                    //
+                    // If we are syncing, there is no point keeping these peers around and
+                    // continually failing to request blocks. We instantly ban them and hope that
+                    // by the time the ban lifts, the peers will have completed their backfill
+                    // sync.
+                    //
+                    // TODO: Potentially a more graceful way of handling such peers, would be to
+                    // implement a new sync type which tracks these peers and prevents the sync
+                    // algorithms from requesting blocks from them (at least for a set period of
+                    // time, multiple failures would then lead to a ban).
                     PeerAction::Fatal
                 }
                 RPCResponseErrorCode::ServerError => PeerAction::MidToleranceError,