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/setup.sh

@@ -33,7 +33,8 @@ lcli \
 	--altair-fork-epoch $ALTAIR_FORK_EPOCH \
 	--eth1-id $NETWORK_ID \
 	--eth1-follow-distance 1 \
-	--seconds-per-eth1-block 1 \
+	--seconds-per-slot $SECONDS_PER_SLOT \
+	--seconds-per-eth1-block $SECONDS_PER_ETH1_BLOCK \
 	--force
 
 echo Specification generated at $TESTNET_DIR.