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_attesters.rs

@@ -381,6 +381,16 @@ impl<T: Item, E: EthSpec> AutoPruningEpochContainer<T, E> {
     pub(crate) fn get_lowest_permissible(&self) -> Epoch {
         self.lowest_permissible_epoch
     }
+
+    /// Returns `true` if the given `index` has been stored in `self` at `epoch`.
+    ///
+    /// This is useful for doppelganger detection.
+    pub fn index_seen_at_epoch(&self, index: usize, epoch: Epoch) -> bool {
+        self.items
+            .get(&epoch)
+            .map(|item| item.contains(index))
+            .unwrap_or(false)
+    }
 }
 
 /// A container that stores some number of `V` items.