Set safe block hash to justified (#3347)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/3189.

## Proposed Changes

- Always supply the justified block hash as the `safe_block_hash` when calling `forkchoiceUpdated` on the execution engine.
- Refactor the `get_payload` routine to use the new `ForkchoiceUpdateParameters` struct rather than just the `finalized_block_hash`. I think this is a nice simplification and that the old way of computing the `finalized_block_hash` was unnecessary, but if anyone sees reason to keep that approach LMK.

beacon_node/execution_layer/src/test_utils/execution_block_generator.rs

@@ -335,7 +335,9 @@ impl<T: EthSpec> ExecutionBlockGenerator<T> {
         }
 
         let unknown_head_block_hash = !self.blocks.contains_key(&forkchoice_state.head_block_hash);
-        let unknown_safe_block_hash = !self.blocks.contains_key(&forkchoice_state.safe_block_hash);
+        let unknown_safe_block_hash = forkchoice_state.safe_block_hash
+            != ExecutionBlockHash::zero()
+            && !self.blocks.contains_key(&forkchoice_state.safe_block_hash);
         let unknown_finalized_block_hash = forkchoice_state.finalized_block_hash
             != ExecutionBlockHash::zero()
             && !self

beacon_node/execution_layer/src/test_utils/mock_execution_layer.rs

@@ -88,11 +88,16 @@ impl<T: EthSpec> MockExecutionLayer<T> {
         let block_number = latest_execution_block.block_number() + 1;
         let timestamp = block_number;
         let prev_randao = Hash256::from_low_u64_be(block_number);
-        let finalized_block_hash = parent_hash;
+        let head_block_root = Hash256::repeat_byte(42);
+        let forkchoice_update_params = ForkchoiceUpdateParameters {
+            head_root: head_block_root,
+            head_hash: Some(parent_hash),
+            justified_hash: None,
+            finalized_hash: None,
+        };
 
         // Insert a proposer to ensure the fork choice updated command works.
         let slot = Slot::new(0);
-        let head_block_root = Hash256::repeat_byte(42);
         let validator_index = 0;
         self.el
             .insert_proposer(
@@ -111,6 +116,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
             .notify_forkchoice_updated(
                 parent_hash,
                 ExecutionBlockHash::zero(),
+                ExecutionBlockHash::zero(),
                 slot,
                 head_block_root,
             )
@@ -124,10 +130,10 @@ impl<T: EthSpec> MockExecutionLayer<T> {
                 parent_hash,
                 timestamp,
                 prev_randao,
-                finalized_block_hash,
                 validator_index,
                 None,
                 slot,
+                forkchoice_update_params,
             )
             .await
             .unwrap()
@@ -148,6 +154,7 @@ impl<T: EthSpec> MockExecutionLayer<T> {
             .notify_forkchoice_updated(
                 block_hash,
                 ExecutionBlockHash::zero(),
+                ExecutionBlockHash::zero(),
                 slot,
                 head_block_root,
             )