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/client/src/builder.rs

@@ -126,9 +126,9 @@ where
         let graffiti = config.graffiti;
 
         let store = store.ok_or("beacon_chain_start_method requires a store")?;
-        let context = runtime_context
-            .ok_or("beacon_chain_start_method requires a runtime context")?
-            .service_context("beacon".into());
+        let runtime_context =
+            runtime_context.ok_or("beacon_chain_start_method requires a runtime context")?;
+        let context = runtime_context.service_context("beacon".into());
         let spec = chain_spec.ok_or("beacon_chain_start_method requires a chain spec")?;
         let event_handler = if self.http_api_config.enabled {
             Some(ServerSentEventHandler::new(context.log().clone()))
@@ -144,7 +144,15 @@ where
             .chain_config(chain_config)
             .disabled_forks(disabled_forks)
             .graffiti(graffiti)
-            .event_handler(event_handler);
+            .event_handler(event_handler)
+            .monitor_validators(
+                config.validator_monitor_auto,
+                config.validator_monitor_pubkeys.clone(),
+                runtime_context
+                    .service_context("val_mon".to_string())
+                    .log()
+                    .clone(),
+            );
 
         let builder = if let Some(slasher) = self.slasher.clone() {
             builder.slasher(slasher)