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>

common/eth2/src/lib.rs

@@ -114,6 +114,7 @@ pub struct Timeouts {
     pub sync_duties: Duration,
     pub get_beacon_blocks_ssz: Duration,
     pub get_debug_beacon_states: Duration,
+    pub get_deposit_snapshot: Duration,
 }
 
 impl Timeouts {
@@ -128,6 +129,7 @@ impl Timeouts {
             sync_duties: timeout,
             get_beacon_blocks_ssz: timeout,
             get_debug_beacon_states: timeout,
+            get_deposit_snapshot: timeout,
         }
     }
 }
@@ -913,6 +915,20 @@ impl BeaconNodeHttpClient {
         Ok(())
     }
 
+    /// `GET beacon/deposit_snapshot`
+    pub async fn get_deposit_snapshot(&self) -> Result<Option<types::DepositTreeSnapshot>, Error> {
+        use ssz::Decode;
+        let mut path = self.eth_path(V1)?;
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("beacon")
+            .push("deposit_snapshot");
+        self.get_bytes_opt_accept_header(path, Accept::Ssz, self.timeouts.get_deposit_snapshot)
+            .await?
+            .map(|bytes| DepositTreeSnapshot::from_ssz_bytes(&bytes).map_err(Error::InvalidSsz))
+            .transpose()
+    }
+
     /// `POST validator/contribution_and_proofs`
     pub async fn post_validator_contribution_and_proofs<T: EthSpec>(
         &self,

common/eth2/src/lighthouse.rs

@@ -6,7 +6,10 @@ mod block_rewards;
 
 use crate::{
     ok_or_error,
-    types::{BeaconState, ChainSpec, Epoch, EthSpec, GenericResponse, ValidatorId},
+    types::{
+        BeaconState, ChainSpec, DepositTreeSnapshot, Epoch, EthSpec, FinalizedExecutionBlock,
+        GenericResponse, ValidatorId,
+    },
     BeaconNodeHttpClient, DepositData, Error, Eth1Data, Hash256, StateId, StatusCode,
 };
 use proto_array::core::ProtoArray;
@@ -331,6 +334,19 @@ impl Eth1Block {
     }
 }
 
+impl From<Eth1Block> for FinalizedExecutionBlock {
+    fn from(eth1_block: Eth1Block) -> Self {
+        Self {
+            deposit_count: eth1_block.deposit_count.unwrap_or(0),
+            deposit_root: eth1_block
+                .deposit_root
+                .unwrap_or_else(|| DepositTreeSnapshot::default().deposit_root),
+            block_hash: eth1_block.hash,
+            block_height: eth1_block.number,
+        }
+    }
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 pub struct DatabaseInfo {
     pub schema_version: u64,