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/network/src/metrics.rs

@@ -338,10 +338,18 @@ lazy_static! {
         "beacon_processor_chain_segment_success_total",
         "Total number of chain segments successfully processed."
     );
+    pub static ref BEACON_PROCESSOR_BACKFILL_CHAIN_SEGMENT_SUCCESS_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_backfill_chain_segment_success_total",
+        "Total number of chain segments successfully processed."
+    );
     pub static ref BEACON_PROCESSOR_CHAIN_SEGMENT_FAILED_TOTAL: Result<IntCounter> = try_create_int_counter(
         "beacon_processor_chain_segment_failed_total",
         "Total number of chain segments that failed processing."
     );
+    pub static ref BEACON_PROCESSOR_BACKFILL_CHAIN_SEGMENT_FAILED_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_backfill_chain_segment_failed_total",
+        "Total number of backfill chain segments that failed processing."
+    );
     // Unaggregated attestations.
     pub static ref BEACON_PROCESSOR_UNAGGREGATED_ATTESTATION_QUEUE_TOTAL: Result<IntGauge> = try_create_int_gauge(
         "beacon_processor_unaggregated_attestation_queue_total",