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/test_utils.rs

@@ -198,7 +198,11 @@ impl<E: EthSpec> BeaconChainHarness<EphemeralHarnessType<E>> {
             .expect("should configure testing slot clock")
             .shutdown_sender(shutdown_tx)
             .chain_config(chain_config)
-            .event_handler(Some(ServerSentEventHandler::new_with_capacity(log, 1)))
+            .event_handler(Some(ServerSentEventHandler::new_with_capacity(
+                log.clone(),
+                1,
+            )))
+            .monitor_validators(true, vec![], log)
             .build()
             .expect("should build");
 
@@ -243,6 +247,7 @@ impl<E: EthSpec> BeaconChainHarness<DiskHarnessType<E>> {
             .testing_slot_clock(HARNESS_SLOT_TIME)
             .expect("should configure testing slot clock")
             .shutdown_sender(shutdown_tx)
+            .monitor_validators(true, vec![], log)
             .build()
             .expect("should build");
 
@@ -284,6 +289,7 @@ impl<E: EthSpec> BeaconChainHarness<DiskHarnessType<E>> {
             .testing_slot_clock(Duration::from_secs(1))
             .expect("should configure testing slot clock")
             .shutdown_sender(shutdown_tx)
+            .monitor_validators(true, vec![], log)
             .build()
             .expect("should build");