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>

book/src/api-lighthouse.md

@@ -353,4 +353,58 @@ curl -X POST "http://localhost:5052/lighthouse/liveness" -d '{"indices":["0","1"
         }
     ]
 }
-```
+```
+
+### `/lighthouse/database/info`
+
+Information about the database's split point and anchor info.
+
+```bash
+curl "http://localhost:5052/lighthouse/database/info" | jq
+```
+
+```json
+{
+  "schema_version": 5,
+  "split": {
+    "slot": "2034912",
+    "state_root": "0x11c8516aa7d4d1613e84121e3a557ceca34618b4c1a38f05b66ad045ff82b33b"
+  },
+  "anchor": {
+    "anchor_slot": "2034720",
+    "oldest_block_slot": "1958881",
+    "oldest_block_parent": "0x1fd3d855d03e9df28d8a41a0f9cb9d4c540832b3ca1c3e1d7e09cd75b874cc87",
+    "state_upper_limit": "2035712",
+    "state_lower_limit": "0"
+  }
+}
+```
+
+For more information about the split point, see the [Database Configuration](./advanced_database.md)
+docs.
+
+The `anchor` will be `null` unless the node has been synced with checkpoint sync and state
+reconstruction has yet to be completed. For more information
+on the specific meanings of these fields see the docs on [Checkpoint
+Sync](./checkpoint-sync.md#reconstructing-states).
+
+### `/lighthouse/database/reconstruct`
+
+Instruct Lighthouse to begin reconstructing historic states, see
+[Reconstructing States](./checkpoint-sync.md#reconstructing-states). This is an alternative
+to the `--reconstruct-historic-states` flag.
+
+```
+curl -X POST "http://localhost:5052/lighthouse/database/reconstruct" | jq
+```
+
+```json
+"success"
+```
+
+The endpoint will return immediately. See the beacon node logs for an indication of progress.
+
+### `/lighthouse/database/historical_blocks`
+
+Manually provide `SignedBeaconBlock`s to backfill the database. This is intended
+for use by Lighthouse developers during testing only.