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>

scripts/local_testnet/ganache_test_node.sh

@@ -2,12 +2,12 @@
 
 source ./vars.env
 
-ganache-cli \
+exec ganache-cli \
 	--defaultBalanceEther 1000000000 \
 	--gasLimit 1000000000 \
 	--accounts 10 \
 	--mnemonic "$ETH1_NETWORK_MNEMONIC" \
 	--port 8545 \
-	--blockTime 3 \
+	--blockTime $SECONDS_PER_ETH1_BLOCK \
 	--networkId "$NETWORK_ID" \
 	--chainId "$NETWORK_ID"