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.

common/eth2/src/lighthouse.rs

@@ -153,7 +153,7 @@ pub struct Eth1SyncStatusData {
     pub head_block_timestamp: Option<u64>,
     pub latest_cached_block_number: Option<u64>,
     pub latest_cached_block_timestamp: Option<u64>,
-    pub voting_period_start_timestamp: u64,
+    pub voting_target_timestamp: u64,
     pub eth1_node_sync_status_percentage: f64,
     pub lighthouse_is_cached_and_ready: bool,
 }