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/config.rs

@@ -24,8 +24,6 @@ pub struct StoreConfig {
 #[derive(Debug, Clone, PartialEq, Eq, Encode, Decode)]
 pub struct OnDiskStoreConfig {
     pub slots_per_restore_point: u64,
-    // NOTE: redundant, see https://github.com/sigp/lighthouse/issues/1784
-    pub _block_cache_size: usize,
 }
 
 #[derive(Debug, Clone)]
@@ -49,7 +47,6 @@ impl StoreConfig {
     pub fn as_disk_config(&self) -> OnDiskStoreConfig {
         OnDiskStoreConfig {
             slots_per_restore_point: self.slots_per_restore_point,
-            _block_cache_size: DEFAULT_BLOCK_CACHE_SIZE,
         }
     }