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/beacon_chain/src/block_verification.rs

@@ -207,6 +207,13 @@ pub enum BlockError<T: EthSpec> {
     /// We were unable to process this block due to an internal error. It's unclear if the block is
     /// valid.
     BeaconChainError(BeaconChainError),
+    /// There was an error whilst verifying weak subjectivity. This block conflicts with the
+    /// configured weak subjectivity checkpoint and was not imported.
+    ///
+    /// ## Peer scoring
+    ///
+    /// The block is invalid and the peer is faulty.
+    WeakSubjectivityConflict,
 }
 
 impl<T: EthSpec> std::fmt::Display for BlockError<T> {