Weak subjectivity start from genesis (#1675)

This commit was edited by Paul H when rebasing from master to
v0.3.0-staging.

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

- 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.

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

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());