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/client/src/builder.rs

@@ -438,6 +438,12 @@ where
 {
     /// Consumes the internal `BeaconChainBuilder`, attaching the resulting `BeaconChain` to self.
     pub fn build_beacon_chain(mut self) -> Result<Self, String> {
+        let context = self
+            .runtime_context
+            .as_ref()
+            .ok_or_else(|| "beacon_chain requires a runtime context")?
+            .clone();
+
         let chain = self
             .beacon_chain_builder
             .ok_or_else(|| "beacon_chain requires a beacon_chain_builder")?
@@ -450,6 +456,7 @@ where
                     .clone()
                     .ok_or_else(|| "beacon_chain requires a slot clock")?,
             )
+            .shutdown_sender(context.executor.shutdown_sender())
             .build()
             .map_err(|e| format!("Failed to build beacon chain: {}", e))?;