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/src/config.rs

@@ -272,6 +272,7 @@ pub fn get_config<E: EthSpec>(
     client_config.eth1.follow_distance = spec.eth1_follow_distance;
     client_config.eth1.network_id = spec.deposit_network_id.into();
     client_config.eth1.chain_id = spec.deposit_chain_id.into();
+    client_config.eth1.set_block_cache_truncation::<E>(spec);
 
     if let Some(mut boot_nodes) = eth2_testnet_config.boot_enr {
         client_config.network.boot_nodes_enr.append(&mut boot_nodes)