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/errors.rs

@@ -2,6 +2,7 @@ use crate::attester_cache::Error as AttesterCacheError;
 use crate::beacon_chain::ForkChoiceError;
 use crate::beacon_fork_choice_store::Error as ForkChoiceStoreError;
 use crate::eth1_chain::Error as Eth1ChainError;
+use crate::historical_blocks::HistoricalBlockError;
 use crate::migrate::PruningError;
 use crate::naive_aggregation_pool::Error as NaiveAggregationError;
 use crate::observed_aggregates::Error as ObservedAttestationsError;
@@ -117,6 +118,7 @@ pub enum BeaconChainError {
         block_slot: Slot,
         state_slot: Slot,
     },
+    HistoricalBlockError(HistoricalBlockError),
     InvalidStateForShuffling {
         state_epoch: Epoch,
         shuffling_epoch: Epoch,
@@ -150,6 +152,7 @@ easy_from_to!(BlockSignatureVerifierError, BeaconChainError);
 easy_from_to!(PruningError, BeaconChainError);
 easy_from_to!(ArithError, BeaconChainError);
 easy_from_to!(ForkChoiceStoreError, BeaconChainError);
+easy_from_to!(HistoricalBlockError, BeaconChainError);
 easy_from_to!(StateAdvanceError, BeaconChainError);
 
 #[derive(Debug)]