Optimise block_root computation in gossip verification

[pawanjay176]

Issue Addressed

N/A

Proposed Changes

Before doing gossip verification, we get the header corresponding to a block to pass to a slasher.
The SignedBeaconBlock and its correspondingSignedBeaconBlockHeader have the same canonical root for the message, however, computing the root for the header is way faster since the root for the tree rooted at BeaconBlockBody is already calculated in the header.

In this PR, we reuse the header to compute the block_root instead of using the full block.
The root computation in mainnet seems to take 10ms on average looking at the beacon_block_processing_block_root_seconds metric.
Using the header to compute the root should take micro seconds so this seems like a low hanging fruit optimisation.

[michaelsproul]

I was wondering if we could delete get_block_root but it looks like it's used in RpcBlock, so happy to leave it as-is for now.

Aside: I'm not super familiar with RpcBlock but have we checked that blocks that fail availability checks are passed to the slasher?

[pawanjay176]

blocks are added to the slasher on calling BeaconChain::process_block which calls IntoExecutionPendingBlock::into_execution_pending_block here

lighthouse/beacon_node/beacon_chain/src/block_verification.rs

Lines 739 to 756 in a3fb27c

	fn into_execution_pending_block(
	self,
	block_root: Hash256,
	chain: &Arc<BeaconChain<T>>,
	notify_execution_layer: NotifyExecutionLayer,
	) -> Result<ExecutionPendingBlock<T>, BlockError<T::EthSpec>> {
	self.into_execution_pending_block_slashable(block_root, chain, notify_execution_layer)
	.map(|execution_pending| {
	// Supply valid block to slasher.
	if let Some(slasher) = chain.slasher.as_ref() {
	slasher.accept_block_header(execution_pending.block.signed_block_header());
	}
	execution_pending
	})
	.map_err(|slash_info| {
	process_block_slash_info::<_, BlockError<T::EthSpec>>(chain, slash_info)
	})
	}

RpcBlock also implements IntoExecutionPendingBlock so it's added to the slasher regardless if it passes or fails the availability checks