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~~

crypto/bls/src/generic_public_key_bytes.rs

@@ -18,12 +18,22 @@ use tree_hash::TreeHash;
 ///
 /// - Lazily verifying a serialized public key.
 /// - Storing some bytes that are actually invalid (required in the case of a `Deposit` message).
-#[derive(Clone)]
 pub struct GenericPublicKeyBytes<Pub> {
     bytes: [u8; PUBLIC_KEY_BYTES_LEN],
     _phantom: PhantomData<Pub>,
 }
 
+impl<Pub> Copy for GenericPublicKeyBytes<Pub> {}
+
+impl<Pub> Clone for GenericPublicKeyBytes<Pub> {
+    fn clone(&self) -> Self {
+        Self {
+            bytes: self.bytes,
+            _phantom: PhantomData,
+        }
+    }
+}
+
 impl<Pub> GenericPublicKeyBytes<Pub>
 where
     Pub: TPublicKey,