Deposit Cache Finalization & Fast WS Sync (#2915)

## Summary

The deposit cache now has the ability to finalize deposits. This will cause it to drop unneeded deposit logs and hashes in the deposit Merkle tree that are no longer required to construct deposit proofs. The cache is finalized whenever the latest finalized checkpoint has a new `Eth1Data` with all deposits imported.

This has three benefits:

1. Improves the speed of constructing Merkle proofs for deposits as we can just replay deposits since the last finalized checkpoint instead of all historical deposits when re-constructing the Merkle tree.
2. Significantly faster weak subjectivity sync as the deposit cache can be transferred to the newly syncing node in compressed form. The Merkle tree that stores `N` finalized deposits requires a maximum of `log2(N)` hashes. The newly syncing node then only needs to download deposits since the last finalized checkpoint to have a full tree.
3. Future proofing in preparation for [EIP-4444](https://eips.ethereum.org/EIPS/eip-4444) as execution nodes will no longer be required to store logs permanently so we won't always have all historical logs available to us.

## More Details

Image to illustrate how the deposit contract merkle tree evolves and finalizes along with the resulting `DepositTreeSnapshot`
![image](https://user-images.githubusercontent.com/37123614/151465302-5fc56284-8a69-4998-b20e-45db3934ac70.png)

## Other Considerations

I've changed the structure of the `SszDepositCache` so once you load & save your database from this version of lighthouse, you will no longer be able to load it from older versions.

Co-authored-by: ethDreamer <37123614+ethDreamer@users.noreply.github.com>

beacon_node/client/src/builder.rs

@@ -277,8 +277,52 @@ where
                     BeaconNodeHttpClient::new(url, Timeouts::set_all(CHECKPOINT_SYNC_HTTP_TIMEOUT));
                 let slots_per_epoch = TEthSpec::slots_per_epoch();
 
-                debug!(context.log(), "Downloading finalized block");
+                let deposit_snapshot = if config.sync_eth1_chain {
+                    // We want to fetch deposit snapshot before fetching the finalized beacon state to
+                    // ensure that the snapshot is not newer than the beacon state that satisfies the
+                    // deposit finalization conditions
+                    debug!(context.log(), "Downloading deposit snapshot");
+                    let deposit_snapshot_result = remote
+                        .get_deposit_snapshot()
+                        .await
+                        .map_err(|e| match e {
+                            ApiError::InvalidSsz(e) => format!(
+                                "Unable to parse SSZ: {:?}. Ensure the checkpoint-sync-url refers to a \
+                                node for the correct network",
+                                e
+                            ),
+                            e => format!("Error fetching deposit snapshot from remote: {:?}", e),
+                        });
+                    match deposit_snapshot_result {
+                        Ok(Some(deposit_snapshot)) => {
+                            if deposit_snapshot.is_valid() {
+                                Some(deposit_snapshot)
+                            } else {
+                                warn!(context.log(), "Remote BN sent invalid deposit snapshot!");
+                                None
+                            }
+                        }
+                        Ok(None) => {
+                            warn!(
+                                context.log(),
+                                "Remote BN does not support EIP-4881 fast deposit sync"
+                            );
+                            None
+                        }
+                        Err(e) => {
+                            warn!(
+                                context.log(),
+                                "Remote BN does not support EIP-4881 fast deposit sync";
+                                "error" => e
+                            );
+                            None
+                        }
+                    }
+                } else {
+                    None
+                };
 
+                debug!(context.log(), "Downloading finalized block");
                 // Find a suitable finalized block on an epoch boundary.
                 let mut block = remote
                     .get_beacon_blocks_ssz::<TEthSpec>(BlockId::Finalized, &spec)
@@ -362,9 +406,33 @@ where
                     "state_root" => ?state_root,
                 );
 
+                let service =
+                    deposit_snapshot.and_then(|snapshot| match Eth1Service::from_deposit_snapshot(
+                        config.eth1,
+                        context.log().clone(),
+                        spec,
+                        &snapshot,
+                    ) {
+                        Ok(service) => {
+                            info!(
+                                context.log(),
+                                "Loaded deposit tree snapshot";
+                                "deposits loaded" => snapshot.deposit_count,
+                            );
+                            Some(service)
+                        }
+                        Err(e) => {
+                            warn!(context.log(),
+                                "Unable to load deposit snapshot";
+                                "error" => ?e
+                            );
+                            None
+                        }
+                    });
+
                 builder
                     .weak_subjectivity_state(state, block, genesis_state)
-                    .map(|v| (v, None))?
+                    .map(|v| (v, service))?
             }
             ClientGenesis::DepositContract => {
                 info!(
@@ -810,9 +878,16 @@ where
         self.freezer_db_path = Some(cold_path.into());
 
         let inner_spec = spec.clone();
+        let deposit_contract_deploy_block = context
+            .eth2_network_config
+            .as_ref()
+            .map(|config| config.deposit_contract_deploy_block)
+            .unwrap_or(0);
+
         let schema_upgrade = |db, from, to| {
             migrate_schema::<Witness<TSlotClock, TEth1Backend, _, _, _>>(
                 db,
+                deposit_contract_deploy_block,
                 datadir,
                 from,
                 to,