Expand eth1 block cache, add more logs (#1938)

## Issue Addressed

NA

## Proposed Changes

- Caches later blocks than is required by `ETH1_FOLLOW_DISTANCE`.
- Adds logging to `warn` if the eth1 cache is insufficiently primed.
- Use `max_by_key` instead of `max_by` in `BeaconChain::Eth1Chain` since it's simpler.
- Rename `voting_period_start_timestamp` to `voting_target_timestamp` for accuracy.

## Additional Info

The reason for eating into the `ETH1_FOLLOW_DISTANCE` and caching blocks that are closer to the head is due to possibility for `SECONDS_PER_ETH1_BLOCK` to be incorrect (as is the case for the Pyrmont testnet on Goerli).

If `SECONDS_PER_ETH1_BLOCK` is too short, we'll skip back too far from the head and skip over blocks that would be valid [`is_candidate_block`](https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/validator.md#eth1-data) blocks. This was the case on the Pyrmont testnet and resulted in Lighthouse choosing blocks that were about 30 minutes older than is ideal.

beacon_node/client/src/notifier.rs

@@ -171,6 +171,38 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
                     "current_slot" => current_slot,
                 );
             }
+
+            // Perform some logging about the eth1 chain
+            if let Some(eth1_chain) = beacon_chain.eth1_chain.as_ref() {
+                if let Some(status) =
+                    eth1_chain.sync_status(head_info.genesis_time, current_slot, &beacon_chain.spec)
+                {
+                    debug!(
+                        log,
+                        "Eth1 cache sync status";
+                        "eth1_head_block" => status.head_block_number,
+                        "latest_cached_block_number" => status.latest_cached_block_number,
+                        "latest_cached_timestamp" => status.latest_cached_block_timestamp,
+                        "voting_target_timestamp" => status.voting_target_timestamp,
+                        "ready" => status.lighthouse_is_cached_and_ready
+                    );
+
+                    if !status.lighthouse_is_cached_and_ready {
+                        warn!(
+                            log,
+                            "Syncing eth1 block cache";
+                            "target_timestamp" => status.voting_target_timestamp,
+                            "latest_timestamp" => status.latest_cached_block_timestamp,
+                            "msg" => "block production temporarily impaired"
+                        );
+                    }
+                } else {
+                    error!(
+                        log,
+                        "Unable to determine eth1 sync status";
+                    );
+                }
+            }
         }
         Ok::<(), ()>(())
     };