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/tests/vars.env

@@ -0,0 +1,39 @@
+# Base directories for the validator keys and secrets
+DATADIR=~/.lighthouse/local-testnet
+
+# Directory for the eth2 config
+TESTNET_DIR=$DATADIR/testnet
+
+# Mnemonic for the ganache test network
+ETH1_NETWORK_MNEMONIC="vast thought differ pull jewel broom cook wrist tribe word before omit"
+
+# Hardcoded deposit contract based on ETH1_NETWORK_MNEMONIC
+DEPOSIT_CONTRACT_ADDRESS=8c594691c0e592ffa21f153a16ae41db5befcaaa
+
+GENESIS_FORK_VERSION=0x42424242
+
+VALIDATOR_COUNT=80
+GENESIS_VALIDATOR_COUNT=80
+
+# Number of validator client instances that you intend to run
+NODE_COUNT=4
+
+GENESIS_DELAY=0
+
+# Port for P2P communication with bootnode
+BOOTNODE_PORT=4242
+
+# Network ID and Chain ID of local eth1 test network
+NETWORK_ID=4242
+
+# Hard fork configuration
+ALTAIR_FORK_EPOCH=18446744073709551615
+
+# Seconds per Eth2 slot
+SECONDS_PER_SLOT=3
+
+# Seconds per Eth1 block
+SECONDS_PER_ETH1_BLOCK=1
+
+# Enable doppelganger detection
+VC_ARGS=" --enable-doppelganger-protection "