Implement API for block rewards (#2628)

## Proposed Changes

Add an API endpoint for retrieving detailed information about block rewards.

For information on usage see [the docs](https://github.com/sigp/lighthouse/blob/block-rewards-api/book/src/api-lighthouse.md#lighthouseblock_rewards), and the source.

beacon_node/http_api/src/block_rewards.rs

@@ -0,0 +1,80 @@
+use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes, WhenSlotSkipped};
+use eth2::lighthouse::{BlockReward, BlockRewardsQuery};
+use slog::{warn, Logger};
+use state_processing::BlockReplayer;
+use std::sync::Arc;
+use warp_utils::reject::{beacon_chain_error, beacon_state_error, custom_bad_request};
+
+pub fn get_block_rewards<T: BeaconChainTypes>(
+    query: BlockRewardsQuery,
+    chain: Arc<BeaconChain<T>>,
+    log: Logger,
+) -> Result<Vec<BlockReward>, warp::Rejection> {
+    let start_slot = query.start_slot;
+    let end_slot = query.end_slot;
+    let prior_slot = start_slot - 1;
+
+    if start_slot > end_slot || start_slot == 0 {
+        return Err(custom_bad_request(format!(
+            "invalid start and end: {}, {}",
+            start_slot, end_slot
+        )));
+    }
+
+    let end_block_root = chain
+        .block_root_at_slot(end_slot, WhenSlotSkipped::Prev)
+        .map_err(beacon_chain_error)?
+        .ok_or_else(|| custom_bad_request(format!("block at end slot {} unknown", end_slot)))?;
+
+    let blocks = chain
+        .store
+        .load_blocks_to_replay(start_slot, end_slot, end_block_root)
+        .map_err(|e| beacon_chain_error(e.into()))?;
+
+    let state_root = chain
+        .state_root_at_slot(prior_slot)
+        .map_err(beacon_chain_error)?
+        .ok_or_else(|| custom_bad_request(format!("prior state at slot {} unknown", prior_slot)))?;
+
+    let mut state = chain
+        .get_state(&state_root, Some(prior_slot))
+        .and_then(|maybe_state| maybe_state.ok_or(BeaconChainError::MissingBeaconState(state_root)))
+        .map_err(beacon_chain_error)?;
+
+    state
+        .build_all_caches(&chain.spec)
+        .map_err(beacon_state_error)?;
+
+    let mut block_rewards = Vec::with_capacity(blocks.len());
+
+    let block_replayer = BlockReplayer::new(state, &chain.spec)
+        .pre_block_hook(Box::new(|state, block| {
+            // Compute block reward.
+            let block_reward =
+                chain.compute_block_reward(block.message(), block.canonical_root(), state)?;
+            block_rewards.push(block_reward);
+            Ok(())
+        }))
+        .state_root_iter(
+            chain
+                .forwards_iter_state_roots_until(prior_slot, end_slot)
+                .map_err(beacon_chain_error)?,
+        )
+        .no_signature_verification()
+        .minimal_block_root_verification()
+        .apply_blocks(blocks, None)
+        .map_err(beacon_chain_error)?;
+
+    if block_replayer.state_root_miss() {
+        warn!(
+            log,
+            "Block reward state root miss";
+            "start_slot" => start_slot,
+            "end_slot" => end_slot,
+        );
+    }
+
+    drop(block_replayer);
+
+    Ok(block_rewards)
+}

beacon_node/http_api/src/lib.rs

@@ -7,6 +7,7 @@
 
 mod attester_duties;
 mod block_id;
+mod block_rewards;
 mod database;
 mod metrics;
 mod proposer_duties;
@@ -2540,6 +2541,16 @@ pub fn serve<T: BeaconChainTypes>(
             },
         );
 
+    let get_lighthouse_block_rewards = warp::path("lighthouse")
+        .and(warp::path("block_rewards"))
+        .and(warp::query::<eth2::lighthouse::BlockRewardsQuery>())
+        .and(warp::path::end())
+        .and(chain_filter.clone())
+        .and(log_filter.clone())
+        .and_then(|query, chain, log| {
+            blocking_json_task(move || block_rewards::get_block_rewards(query, chain, log))
+        });
+
     let get_events = eth1_v1
         .and(warp::path("events"))
         .and(warp::path::end())
@@ -2576,6 +2587,9 @@ pub fn serve<T: BeaconChainTypes>(
                                 api_types::EventTopic::LateHead => {
                                     event_handler.subscribe_late_head()
                                 }
+                                api_types::EventTopic::BlockReward => {
+                                    event_handler.subscribe_block_reward()
+                                }
                             };
 
                             receivers.push(BroadcastStream::new(receiver).map(|msg| {
@@ -2661,6 +2675,7 @@ pub fn serve<T: BeaconChainTypes>(
                 .or(get_lighthouse_beacon_states_ssz.boxed())
                 .or(get_lighthouse_staking.boxed())
                 .or(get_lighthouse_database_info.boxed())
+                .or(get_lighthouse_block_rewards.boxed())
                 .or(get_events.boxed()),
         )
         .or(warp::post().and(