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/behaviour/mod.rs

@@ -522,7 +522,7 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
     }
 
     /// Inform the peer that their request produced an error.
-    pub fn _send_error_reponse(
+    pub fn send_error_reponse(
         &mut self,
         peer_id: PeerId,
         id: PeerRequestId,