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~~

consensus/types/src/beacon_state.rs

@@ -1013,7 +1013,7 @@ impl<T: EthSpec> BeaconState<T> {
             .enumerate()
             .skip(self.pubkey_cache.len())
         {
-            let success = self.pubkey_cache.insert(validator.pubkey.clone(), i);
+            let success = self.pubkey_cache.insert(validator.pubkey, i);
             if !success {
                 return Err(Error::PubkeyCacheInconsistent);
             }

consensus/types/src/deposit_data.rs

@@ -26,7 +26,7 @@ impl DepositData {
     /// Spec v0.12.1
     pub fn as_deposit_message(&self) -> DepositMessage {
         DepositMessage {
-            pubkey: self.pubkey.clone(),
+            pubkey: self.pubkey,
             withdrawal_credentials: self.withdrawal_credentials,
             amount: self.amount,
         }