Weak subjectivity start from genesis (#1675)

## Issue Addressed
Solution 2 proposed here: https://github.com/sigp/lighthouse/issues/1435#issuecomment-692317639

## Proposed Changes
- Adds an optional `--wss-checkpoint` flag that takes a string `root:epoch`
- Verify that the given checkpoint exists in the chain, or that the the chain syncs through this checkpoint. If not, shutdown and prompt the user to purge state before restarting.

## Additional Info


Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/network/src/attestation_service/tests/mod.rs

@@ -47,6 +47,9 @@ mod tests {
             let store =
                 HotColdDB::open_ephemeral(StoreConfig::default(), spec.clone(), log.clone())
                     .unwrap();
+
+            let (shutdown_tx, _) = futures::channel::mpsc::channel(1);
+
             let chain = Arc::new(
                 BeaconChainBuilder::new(MinimalEthSpec)
                     .logger(log.clone())
@@ -67,6 +70,7 @@ mod tests {
                         Duration::from_secs(recent_genesis_time()),
                         Duration::from_millis(SLOT_DURATION_MILLIS),
                     ))
+                    .shutdown_sender(shutdown_tx)
                     .build()
                     .expect("should build"),
             );

beacon_node/network/src/beacon_processor/worker.rs

@@ -231,6 +231,7 @@ impl<T: BeaconChainTypes> Worker<T> {
             | Err(e @ BlockError::BlockIsNotLaterThanParent { .. })
             | Err(e @ BlockError::InvalidSignature)
             | Err(e @ BlockError::TooManySkippedSlots { .. })
+            | Err(e @ BlockError::WeakSubjectivityConflict)
             | Err(e @ BlockError::GenesisBlock) => {
                 warn!(self.log, "Could not verify block for gossip, rejecting the block";
                             "error" => e.to_string());