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/beacon_chain/src/errors.rs

@@ -5,6 +5,7 @@ use crate::naive_aggregation_pool::Error as NaiveAggregationError;
 use crate::observed_attestations::Error as ObservedAttestationsError;
 use crate::observed_attesters::Error as ObservedAttestersError;
 use crate::observed_block_producers::Error as ObservedBlockProducersError;
+use futures::channel::mpsc::TrySendError;
 use operation_pool::OpPoolError;
 use safe_arith::ArithError;
 use ssz_types::Error as SszTypesError;
@@ -87,6 +88,8 @@ pub enum BeaconChainError {
         shuffling_epoch: Epoch,
         head_block_epoch: Epoch,
     },
+    WeakSubjectivtyVerificationFailure,
+    WeakSubjectivtyShutdownError(TrySendError<&'static str>),
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);