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/chain_config.rs

@@ -12,6 +12,8 @@ pub struct ChainConfig {
     ///
     /// If `None`, there is no weak subjectivity verification.
     pub weak_subjectivity_checkpoint: Option<Checkpoint>,
+    /// Determine whether to reconstruct historic states, usually after a checkpoint sync.
+    pub reconstruct_historic_states: bool,
 }
 
 impl Default for ChainConfig {
@@ -19,6 +21,7 @@ impl Default for ChainConfig {
         Self {
             import_max_skip_slots: None,
             weak_subjectivity_checkpoint: None,
+            reconstruct_historic_states: false,
         }
     }
 }