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/beacon_chain/src/test_utils.rs

@@ -12,7 +12,7 @@ use crate::{
 };
 use bls::get_withdrawal_credentials;
 use futures::channel::mpsc::Receiver;
-use genesis::interop_genesis_state;
+pub use genesis::interop_genesis_state;
 use int_to_bytes::int_to_bytes32;
 use merkle_proof::MerkleTree;
 use parking_lot::Mutex;