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/chunked_vector.rs

@@ -34,8 +34,8 @@ pub enum UpdatePattern {
 /// Map a chunk index to bytes that can be used to key the NoSQL database.
 ///
 /// We shift chunks up by 1 to make room for a genesis chunk that is handled separately.
-pub fn chunk_key(cindex: u64) -> [u8; 8] {
-    (cindex + 1).to_be_bytes()
+pub fn chunk_key(cindex: usize) -> [u8; 8] {
+    (cindex as u64 + 1).to_be_bytes()
 }
 
 /// Return the database key for the genesis value.
@@ -73,6 +73,11 @@ pub trait Field<E: EthSpec>: Copy {
         128
     }
 
+    /// Convert a v-index (vector index) to a chunk index.
+    fn chunk_index(vindex: usize) -> usize {
+        vindex / Self::chunk_size()
+    }
+
     /// Get the value of this field at the given vector index, from the state.
     fn get_value(
         state: &BeaconState<E>,
@@ -399,7 +404,7 @@ where
     I: Iterator<Item = usize>,
 {
     for chunk_index in range {
-        let chunk_key = &chunk_key(chunk_index as u64)[..];
+        let chunk_key = &chunk_key(chunk_index)[..];
 
         let existing_chunk =
             Chunk::<F::Value>::load(store, F::column(), chunk_key)?.unwrap_or_else(Chunk::default);
@@ -440,7 +445,7 @@ fn range_query<S: KeyValueStore<E>, E: EthSpec, T: Decode + Encode>(
     let mut result = Vec::with_capacity(len);
 
     for chunk_index in range {
-        let key = &chunk_key(chunk_index as u64)[..];
+        let key = &chunk_key(chunk_index)[..];
         let chunk = Chunk::load(store, column, key)?.ok_or(ChunkError::Missing { chunk_index })?;
         result.push(chunk);
     }