feat!: LocalChain with hardwired genesis checkpoint

This ensures that `LocalChain` will always have a tip. The `ChainOracle`
trait's `get_chain_tip` method no longer needs to return an option.

crates/chain/src/chain_oracle.rs

@@ -21,5 +21,5 @@ pub trait ChainOracle {
     ) -> Result<Option<bool>, Self::Error>;
 
     /// Get the best chain's chain tip.
-    fn get_chain_tip(&self) -> Result<Option<BlockId>, Self::Error>;
+    fn get_chain_tip(&self) -> Result<BlockId, Self::Error>;
 }