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>

validator_client/src/http_metrics/mod.rs

@@ -35,7 +35,7 @@ impl From<String> for Error {
 
 /// Contains objects which have shared access from inside/outside of the metrics server.
 pub struct Shared<T: EthSpec> {
-    pub validator_store: Option<ValidatorStore<SystemTimeSlotClock, T>>,
+    pub validator_store: Option<Arc<ValidatorStore<SystemTimeSlotClock, T>>>,
     pub duties_service: Option<Arc<DutiesService<SystemTimeSlotClock, T>>>,
     pub genesis_time: Option<u64>,
 }