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/observed_block_producers.rs

@@ -3,7 +3,7 @@
 
 use std::collections::{HashMap, HashSet};
 use std::marker::PhantomData;
-use types::{BeaconBlockRef, EthSpec, Slot, Unsigned};
+use types::{BeaconBlockRef, Epoch, EthSpec, Slot, Unsigned};
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
@@ -114,6 +114,15 @@ impl<E: EthSpec> ObservedBlockProducers<E> {
         self.finalized_slot = finalized_slot;
         self.items.retain(|slot, _set| *slot > finalized_slot);
     }
+
+    /// Returns `true` if the given `validator_index` has been stored in `self` at `epoch`.
+    ///
+    /// This is useful for doppelganger detection.
+    pub fn index_seen_at_epoch(&self, validator_index: u64, epoch: Epoch) -> bool {
+        self.items.iter().any(|(slot, producers)| {
+            slot.epoch(E::slots_per_epoch()) == epoch && producers.contains(&validator_index)
+        })
+    }
 }
 
 #[cfg(test)]