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/rpc/codec/base.rs

@@ -211,16 +211,13 @@ mod tests {
         let _ = snappy_buf.split_to(1);
 
         // decode message just as snappy message
-        let snappy_decoded_message = snappy_outbound_codec.decode(&mut snappy_buf).unwrap();
+        let _snappy_decoded_message = snappy_outbound_codec.decode(&mut snappy_buf).unwrap();
 
         // build codecs for entire chunk
         let mut snappy_base_outbound_codec = BaseOutboundCodec::new(snappy_outbound_codec);
 
         // decode message as ssz snappy chunk
-        let snappy_decoded_chunk = snappy_base_outbound_codec.decode(&mut buf).unwrap();
-
-        dbg!(snappy_decoded_message);
-        dbg!(snappy_decoded_chunk);
+        let _snappy_decoded_chunk = snappy_base_outbound_codec.decode(&mut buf).unwrap();
     }
 
     #[test]