Fix and test checkpoint sync from genesis (#7689)

Fix a bug involving checkpoint sync from genesis reported by Sunnyside labs.


  Ensure that the store's `anchor` is initialised prior to storing the genesis state. In the case of checkpoint sync from genesis, the genesis state will be in the _hot DB_, so we need the hot DB metadata to be initialised in order to store it.

I've extended the existing checkpoint sync tests to cover this case as well. There are some subtleties around what the `state_upper_limit` should be set to in this case. I've opted to just enable state reconstruction from the start in the test so it gets set to 0, which results in an end state more consistent with the other test cases (full state reconstruction). This is required because we can't meaningfully do any state reconstruction when the split slot is 0 (there is no range of frozen slots to reconstruct).

beacon_node/store/src/reconstruct.rs

@@ -47,6 +47,12 @@ where
         let lower_limit_slot = anchor.state_lower_limit;
         let upper_limit_slot = std::cmp::min(split.slot, anchor.state_upper_limit);
 
+        // If the split is at 0 we can't reconstruct historic states.
+        if split.slot == 0 {
+            debug!("No state reconstruction possible");
+            return Ok(());
+        }
+
         // If `num_blocks` is not specified iterate all blocks. Add 1 so that we end on an epoch
         // boundary when `num_blocks` is a multiple of an epoch boundary. We want to be *inclusive*
         // of the state at slot `lower_limit_slot + num_blocks`.