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/state_processing/src/verify_operation.rs

@@ -20,6 +20,10 @@ impl<T> SigVerifiedOp<T> {
     pub fn into_inner(self) -> T {
         self.0
     }
+
+    pub fn as_inner(&self) -> &T {
+        &self.0
+    }
 }
 
 /// Trait for operations that can be verified and transformed into a `SigVerifiedOp`.