Ensure FC uses the current slot from the store (#3402)

## Issue Addressed

NA

## Proposed Changes

Ensure that we read the current slot from the `fc_store` rather than the slot clock. This is because the `fc_store` will never allow the slot to go backwards, even if the system clock does. The `ProtoArray::find_head` function assumes a non-decreasing slot.

This issue can cause logs like this:

```
ERRO Error whist recomputing head, error: ForkChoiceError(ProtoArrayError("find_head failed: InvalidBestNode(InvalidBestNodeInfo { start_root: 0xb22655aa2ae23075a60bd40797b3ba220db33d6fb86fa7910f0ed48e34bda72f, justified_checkpoint: Checkpoint { epoch: Epoch(111569), root: 0xb22655aa2ae23075a60bd40797b3ba220db33d6fb86fa7910f0ed48e34bda72f }, finalized_checkpoint: Checkpoint { epoch: Epoch(111568), root: 0x6140797e40c587b0d3f159483bbc603accb7b3af69891979d63efac437f9896f }, head_root: 0xb22655aa2ae23075a60bd40797b3ba220db33d6fb86fa7910f0ed48e34bda72f, head_justified_checkpoint: Some(Checkpoint { epoch: Epoch(111568), root: 0x6140797e40c587b0d3f159483bbc603accb7b3af69891979d63efac437f9896f }), head_finalized_checkpoint: Some(Checkpoint { epoch: Epoch(111567), root: 0x59b913d37383a158a9ea5546a572acc79e2cdfbc904c744744789d2c3814c570 }) })")), service: beacon, module: beacon_chain::canonical_head:499
```

We expect nodes to automatically recover from this issue within seconds without any major impact. However, having *any* errors in the path of fork choice is undesirable and should be avoided.

## Additional Info

NA

consensus/fork_choice/src/fork_choice.rs

@@ -485,10 +485,13 @@ where
     /// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/fork-choice.md#get_head
     pub fn get_head(
         &mut self,
-        current_slot: Slot,
+        system_time_current_slot: Slot,
         spec: &ChainSpec,
     ) -> Result<Hash256, Error<T::Error>> {
-        self.update_time(current_slot, spec)?;
+        // Provide the slot (as per the system clock) to the `fc_store` and then return its view of
+        // the current slot. The `fc_store` will ensure that the `current_slot` is never
+        // decreasing, a property which we must maintain.
+        let current_slot = self.update_time(system_time_current_slot, spec)?;
 
         let store = &mut self.fc_store;
 
@@ -639,7 +642,7 @@ where
     #[allow(clippy::too_many_arguments)]
     pub fn on_block<Payload: ExecPayload<E>>(
         &mut self,
-        current_slot: Slot,
+        system_time_current_slot: Slot,
         block: BeaconBlockRef<E, Payload>,
         block_root: Hash256,
         block_delay: Duration,
@@ -648,7 +651,10 @@ where
         spec: &ChainSpec,
         count_unrealized: CountUnrealized,
     ) -> Result<(), Error<T::Error>> {
-        let current_slot = self.update_time(current_slot, spec)?;
+        // Provide the slot (as per the system clock) to the `fc_store` and then return its view of
+        // the current slot. The `fc_store` will ensure that the `current_slot` is never
+        // decreasing, a property which we must maintain.
+        let current_slot = self.update_time(system_time_current_slot, spec)?;
 
         // Parent block must be known.
         let parent_block = self
@@ -1051,13 +1057,12 @@ where
     /// will not be run here.
     pub fn on_attestation(
         &mut self,
-        current_slot: Slot,
+        system_time_current_slot: Slot,
         attestation: &IndexedAttestation<E>,
         is_from_block: AttestationFromBlock,
         spec: &ChainSpec,
     ) -> Result<(), Error<T::Error>> {
-        // Ensure the store is up-to-date.
-        self.update_time(current_slot, spec)?;
+        self.update_time(system_time_current_slot, spec)?;
 
         // Ignore any attestations to the zero hash.
         //

consensus/proto_array/src/error.rs

@@ -1,4 +1,4 @@
-use types::{Checkpoint, Epoch, ExecutionBlockHash, Hash256};
+use types::{Checkpoint, Epoch, ExecutionBlockHash, Hash256, Slot};
 
 #[derive(Clone, PartialEq, Debug)]
 pub enum Error {
@@ -52,6 +52,7 @@ pub enum Error {
 
 #[derive(Clone, PartialEq, Debug)]
 pub struct InvalidBestNodeInfo {
+    pub current_slot: Slot,
     pub start_root: Hash256,
     pub justified_checkpoint: Checkpoint,
     pub finalized_checkpoint: Checkpoint,

consensus/proto_array/src/proto_array.rs

@@ -659,6 +659,7 @@ impl ProtoArray {
         // Perform a sanity check that the node is indeed valid to be the head.
         if !self.node_is_viable_for_head::<E>(best_node, current_slot) {
             return Err(Error::InvalidBestNode(Box::new(InvalidBestNodeInfo {
+                current_slot,
                 start_root: *justified_root,
                 justified_checkpoint: self.justified_checkpoint,
                 finalized_checkpoint: self.finalized_checkpoint,