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>

book/src/api-lighthouse.md

@@ -329,3 +329,28 @@ curl -X GET "http://localhost:5052/lighthouse/beacon/states/0/ssz" | jq
 ```
 
 *Example omitted for brevity, the body simply contains SSZ bytes.*
+
+### `/lighthouse/liveness`
+
+POST request that checks if any of the given validators have attested in the given epoch. Returns a list
+of objects, each including the validator index, epoch, and `is_live` status of a requested validator.
+
+This endpoint is used in doppelganger detection, and will only provide accurate information for the
+current, previous, or next epoch.
+
+
+```bash
+curl -X POST "http://localhost:5052/lighthouse/liveness" -d '{"indices":["0","1"],"epoch":"1"}' -H  "content-type: application/json" | jq
+```
+
+```json
+{
+    "data": [
+        {
+            "index": "0",
+            "epoch": "1",
+            "is_live": true
+        }
+    ]
+}
+```