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/lib.rs

@@ -59,6 +59,9 @@ pub trait SlotClock: Send + Sync + Sized {
     /// Returns the duration until the first slot of the next epoch.
     fn duration_to_next_epoch(&self, slots_per_epoch: u64) -> Option<Duration>;
 
+    /// Returns the start time of the slot, as a duration since `UNIX_EPOCH`.
+    fn start_of(&self, slot: Slot) -> Option<Duration>;
+
     /// Returns the first slot to be returned at the genesis time.
     fn genesis_slot(&self) -> Slot;