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>

.github/workflows/test-suite.yml

@@ -146,6 +146,28 @@ jobs:
         run: sudo npm install -g ganache-cli
       - name: Run the syncing simulator
         run: cargo run --release --bin simulator syncing-sim
+  doppelganger-protection-test:
+      name: doppelganger-protection-test
+      runs-on: ubuntu-latest
+      needs: cargo-fmt
+      steps:
+          - uses: actions/checkout@v1
+          - name: Get latest version of stable Rust
+            run: rustup update stable
+          - name: Install ganache-cli
+            run: sudo npm install -g ganache-cli
+          - name: Install lighthouse and lcli
+            run: |
+                make
+                make install-lcli
+          - name: Run the doppelganger protection success test script
+            run: |
+                cd scripts/tests
+                ./doppelganger_protection.sh success
+          - name: Run the doppelganger protection failure test script
+            run: |
+                cd scripts/tests
+                ./doppelganger_protection.sh failure
   check-benchmarks:
     name: check-benchmarks
     runs-on: ubuntu-latest