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/cli.rs

@@ -216,4 +216,19 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 and never provide an untrusted URL.")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("enable-doppelganger-protection")
+                .long("enable-doppelganger-protection")
+                .value_name("ENABLE_DOPPELGANGER_PROTECTION")
+                .help("If this flag is set, Lighthouse will delay startup for three epochs and \
+                    monitor for messages on the network by any of the validators managed by this \
+                    client. This will result in three (possibly four) epochs worth of missed \
+                    attestations. If an attestation is detected during this period, it means it is \
+                    very likely that you are running a second validator client with the same keys. \
+                    This validator client will immediately shutdown if this is detected in order \
+                    to avoid potentially committing a slashable offense. Use this flag in order to \
+                    ENABLE this functionality, without this flag Lighthouse will begin attesting \
+                    immediately.")
+                .takes_value(false),
+        )
 }