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>

common/eth2/src/lib.rs

@@ -83,6 +83,7 @@ impl fmt::Display for Error {
 pub struct Timeouts {
     pub attestation: Duration,
     pub attester_duties: Duration,
+    pub liveness: Duration,
     pub proposal: Duration,
     pub proposer_duties: Duration,
 }
@@ -92,6 +93,7 @@ impl Timeouts {
         Timeouts {
             attestation: timeout,
             attester_duties: timeout,
+            liveness: timeout,
             proposal: timeout,
             proposer_duties: timeout,
         }
@@ -1103,6 +1105,30 @@ impl BeaconNodeHttpClient {
             .await
     }
 
+    /// `POST lighthouse/liveness`
+    pub async fn post_lighthouse_liveness(
+        &self,
+        ids: &[u64],
+        epoch: Epoch,
+    ) -> Result<GenericResponse<Vec<LivenessResponseData>>, Error> {
+        let mut path = self.server.full.clone();
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("lighthouse")
+            .push("liveness");
+
+        self.post_with_timeout_and_response(
+            path,
+            &LivenessRequestData {
+                indices: ids.to_vec(),
+                epoch,
+            },
+            self.timeouts.liveness,
+        )
+        .await
+    }
+
     /// `POST validator/duties/attester/{epoch}`
     pub async fn post_validator_duties_attester(
         &self,

common/eth2/src/types.rs

@@ -847,6 +847,21 @@ impl FromStr for Accept {
     }
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+pub struct LivenessRequestData {
+    pub epoch: Epoch,
+    #[serde(with = "serde_utils::quoted_u64_vec")]
+    pub indices: Vec<u64>,
+}
+
+#[derive(PartialEq, Debug, Serialize, Deserialize)]
+pub struct LivenessResponseData {
+    #[serde(with = "serde_utils::quoted_u64")]
+    pub index: u64,
+    pub epoch: Epoch,
+    pub is_live: bool,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;