Deduplicate block root computation (#3590)

## Issue Addressed

NA

## Proposed Changes

This PR removes duplicated block root computation.

Computing the `SignedBeaconBlock::canonical_root` has become more expensive since the merge as we need to compute the merke root of each transaction inside an `ExecutionPayload`.

Computing the root for [a mainnet block](https://beaconcha.in/slot/4704236) is taking ~10ms on my i7-8700K CPU @ 3.70GHz (no sha extensions). Given that our median seen-to-imported time for blocks is presently 300-400ms, removing a few duplicated block roots (~30ms) could represent an easy 10% improvement. When we consider that the seen-to-imported times include operations *after* the block has been placed in the early attester cache, we could expect the 30ms to be more significant WRT our seen-to-attestable times.

## Additional Info

NA

testing/ef_tests/src/cases/fork_choice.rs

@@ -331,11 +331,11 @@ impl<E: EthSpec> Tester<E> {
     pub fn process_block(&self, block: SignedBeaconBlock<E>, valid: bool) -> Result<(), Error> {
         let block_root = block.canonical_root();
         let block = Arc::new(block);
-        let result = self.block_on_dangerous(
-            self.harness
-                .chain
-                .process_block(block.clone(), CountUnrealized::False),
-        )?;
+        let result = self.block_on_dangerous(self.harness.chain.process_block(
+            block_root,
+            block.clone(),
+            CountUnrealized::False,
+        ))?;
         if result.is_ok() != valid {
             return Err(Error::DidntFail(format!(
                 "block with root {} was valid={} whilst test expects valid={}. result: {:?}",