Detailed validator monitoring (#2151)

## Issue Addressed

- Resolves #2064

## Proposed Changes

Adds a `ValidatorMonitor` struct which provides additional logging and Grafana metrics for specific validators.

Use `lighthouse bn --validator-monitor` to automatically enable monitoring for any validator that hits the [subnet subscription](https://ethereum.github.io/eth2.0-APIs/#/Validator/prepareBeaconCommitteeSubnet) HTTP API endpoint.

Also, use `lighthouse bn --validator-monitor-pubkeys` to supply a list of validators which will always be monitored.

See the new docs included in this PR for more info.

## TODO

- [x] Track validator balance, `slashed` status, etc.
- [x] ~~Register slashings in current epoch, not offense epoch~~
- [ ] Publish Grafana dashboard, update TODO link in docs
- [x] ~~#2130 is merged into this branch, resolve that~~

beacon_node/beacon_chain/src/validator_pubkey_cache.rs

@@ -93,7 +93,7 @@ impl ValidatorPubkeyCache {
                     .map_err(BeaconChainError::InvalidValidatorPubkeyBytes)?,
             );
 
-            self.indices.insert(v.pubkey.clone(), i);
+            self.indices.insert(v.pubkey, i);
         }
 
         Ok(())