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/store/src/lib.rs

@@ -10,6 +10,7 @@
 #[macro_use]
 extern crate lazy_static;
 
+mod chunk_writer;
 pub mod chunked_iter;
 pub mod chunked_vector;
 pub mod config;
@@ -23,9 +24,11 @@ mod memory_store;
 pub mod metadata;
 pub mod metrics;
 mod partial_beacon_state;
+pub mod reconstruct;
 
 pub mod iter;
 
+pub use self::chunk_writer::ChunkWriter;
 pub use self::config::StoreConfig;
 pub use self::hot_cold_store::{BlockReplay, HotColdDB, HotStateSummary, Split};
 pub use self::leveldb_store::LevelDB;
@@ -33,6 +36,7 @@ pub use self::memory_store::MemoryStore;
 pub use self::partial_beacon_state::PartialBeaconState;
 pub use errors::Error;
 pub use impls::beacon_state::StorageContainer as BeaconStateStorageContainer;
+pub use metadata::AnchorInfo;
 pub use metrics::scrape_for_metrics;
 use parking_lot::MutexGuard;
 pub use types::*;