Doppelganger detection (#2230)

## Issue Addressed

Resolves #2069 

## Proposed Changes

- Adds a `--doppelganger-detection` flag
- Adds a `lighthouse/seen_validators` endpoint, which will make it so the lighthouse VC is not interopable with other client beacon nodes if the `--doppelganger-detection` flag is used, but hopefully this will become standardized. Relevant Eth2 API repo issue: https://github.com/ethereum/eth2.0-APIs/issues/64
- If the `--doppelganger-detection` flag is used, the VC will wait until the beacon node is synced, and then wait an additional 2 epochs. The reason for this is to make sure the beacon node is able to subscribe to the subnets our validators should be attesting on. I think an alternative would be to have the beacon node subscribe to all subnets for 2+ epochs on startup by default.

## Additional Info

I'd like to add tests and would appreciate feedback. 

TODO:  handle validators started via the API, potentially make this default behavior

Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -3443,6 +3443,28 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         let mut file = std::fs::File::create(file_name).unwrap();
         self.dump_as_dot(&mut file);
     }
+
+    /// Checks if attestations have been seen from the given `validator_index` at the
+    /// given `epoch`.
+    pub fn validator_seen_at_epoch(&self, validator_index: usize, epoch: Epoch) -> bool {
+        // It's necessary to assign these checks to intermediate variables to avoid a deadlock.
+        //
+        // See: https://github.com/sigp/lighthouse/pull/2230#discussion_r620013993
+        let attested = self
+            .observed_attesters
+            .read()
+            .index_seen_at_epoch(validator_index, epoch);
+        let aggregated = self
+            .observed_aggregators
+            .read()
+            .index_seen_at_epoch(validator_index, epoch);
+        let produced_block = self
+            .observed_block_producers
+            .read()
+            .index_seen_at_epoch(validator_index as u64, epoch);
+
+        attested || aggregated || produced_block
+    }
 }
 
 impl<T: BeaconChainTypes> Drop for BeaconChain<T> {