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>

common/eth2/src/lighthouse.rs

@@ -9,6 +9,7 @@ use proto_array::core::ProtoArray;
 use reqwest::IntoUrl;
 use serde::{Deserialize, Serialize};
 use ssz_derive::{Decode, Encode};
+use store::{AnchorInfo, Split};
 
 pub use eth2_libp2p::{types::SyncState, PeerInfo};
 
@@ -311,6 +312,13 @@ impl Eth1Block {
     }
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+pub struct DatabaseInfo {
+    pub schema_version: u64,
+    pub split: Split,
+    pub anchor: Option<AnchorInfo>,
+}
+
 impl BeaconNodeHttpClient {
     /// Perform a HTTP GET request, returning `None` on a 404 error.
     async fn get_bytes_opt<U: IntoUrl>(&self, url: U) -> Result<Option<Vec<u8>>, Error> {
@@ -490,4 +498,30 @@ impl BeaconNodeHttpClient {
 
         self.get_opt::<(), _>(path).await.map(|opt| opt.is_some())
     }
+
+    /// `GET lighthouse/database/info`
+    pub async fn get_lighthouse_database_info(&self) -> Result<DatabaseInfo, Error> {
+        let mut path = self.server.full.clone();
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("lighthouse")
+            .push("database")
+            .push("info");
+
+        self.get(path).await
+    }
+
+    /// `POST lighthouse/database/reconstruct`
+    pub async fn post_lighthouse_database_reconstruct(&self) -> Result<String, Error> {
+        let mut path = self.server.full.clone();
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("lighthouse")
+            .push("database")
+            .push("reconstruct");
+
+        self.post_with_response(path, &()).await
+    }
 }