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

beacon_node/network/src/beacon_processor/mod.rs

@@ -489,6 +489,7 @@ impl<T: BeaconChainTypes> WorkEvent<T> {
     /// Create a new `Work` event for some block, where the result from computation (if any) is
     /// sent to the other side of `result_tx`.
     pub fn rpc_beacon_block(
+        block_root: Hash256,
         block: Arc<SignedBeaconBlock<T::EthSpec>>,
         seen_timestamp: Duration,
         process_type: BlockProcessType,
@@ -496,6 +497,7 @@ impl<T: BeaconChainTypes> WorkEvent<T> {
         Self {
             drop_during_sync: false,
             work: Work::RpcBlock {
+                block_root,
                 block,
                 seen_timestamp,
                 process_type,
@@ -577,6 +579,7 @@ impl<T: BeaconChainTypes> std::convert::From<ReadyWork<T>> for WorkEvent<T> {
                 },
             },
             ReadyWork::RpcBlock(QueuedRpcBlock {
+                block_root,
                 block,
                 seen_timestamp,
                 process_type,
@@ -584,6 +587,7 @@ impl<T: BeaconChainTypes> std::convert::From<ReadyWork<T>> for WorkEvent<T> {
             }) => Self {
                 drop_during_sync: false,
                 work: Work::RpcBlock {
+                    block_root,
                     block,
                     seen_timestamp,
                     process_type,
@@ -705,6 +709,7 @@ pub enum Work<T: BeaconChainTypes> {
         seen_timestamp: Duration,
     },
     RpcBlock {
+        block_root: Hash256,
         block: Arc<SignedBeaconBlock<T::EthSpec>>,
         seen_timestamp: Duration,
         process_type: BlockProcessType,
@@ -1532,11 +1537,13 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
              * Verification for beacon blocks received during syncing via RPC.
              */
             Work::RpcBlock {
+                block_root,
                 block,
                 seen_timestamp,
                 process_type,
                 should_process,
             } => task_spawner.spawn_async(worker.process_rpc_block(
+                block_root,
                 block,
                 seen_timestamp,
                 process_type,