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~~

common/slot_clock/src/system_time_slot_clock.rs

@@ -54,6 +54,10 @@ impl SlotClock for SystemTimeSlotClock {
         self.clock.duration_to_slot(slot, now)
     }
 
+    fn start_of(&self, slot: Slot) -> Option<Duration> {
+        self.clock.start_of(slot)
+    }
+
     fn genesis_slot(&self) -> Slot {
         self.clock.genesis_slot()
     }