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>

lighthouse/tests/validator_client.rs

@@ -378,3 +378,16 @@ pub fn malloc_tuning_flag() {
         // effects of it.
         .run();
 }
+#[test]
+fn doppelganger_protection_flag() {
+    CommandLineTest::new()
+        .flag("enable-doppelganger-protection", None)
+        .run()
+        .with_config(|config| assert!(config.enable_doppelganger_protection));
+}
+#[test]
+fn no_doppelganger_protection_flag() {
+    CommandLineTest::new()
+        .run()
+        .with_config(|config| assert!(!config.enable_doppelganger_protection));
+}