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>

lcli/src/main.rs

@@ -344,6 +344,13 @@ fn main() {
                               non-default.",
                         ),
                 )
+                .arg(
+                    Arg::with_name("seconds-per-slot")
+                        .long("seconds-per-slot")
+                        .value_name("SECONDS")
+                        .takes_value(true)
+                        .help("Eth2 slot time"),
+                )
                 .arg(
                     Arg::with_name("seconds-per-eth1-block")
                         .long("seconds-per-eth1-block")

lcli/src/new_testnet.rs

@@ -43,6 +43,7 @@ pub fn run<T: EthSpec>(testnet_dir_path: PathBuf, matches: &ArgMatches) -> Resul
     maybe_update!("genesis-delay", genesis_delay);
     maybe_update!("eth1-id", deposit_chain_id);
     maybe_update!("eth1-id", deposit_network_id);
+    maybe_update!("seconds-per-slot", seconds_per_slot);
     maybe_update!("seconds-per-eth1-block", seconds_per_eth1_block);
 
     if let Some(v) = parse_ssz_optional(matches, "genesis-fork-version")? {