Start heavy refactor of validator client

- Block production is working

eth2/utils/slot_clock/src/lib.rs

@@ -28,4 +28,7 @@ pub trait SlotClock: Send + Sync + Sized {
 
     /// Returns the duration until the next slot.
     fn duration_to_next_slot(&self) -> Option<Duration>;
+
+    /// Returns the duration until the first slot of the next epoch.
+    fn duration_to_next_epoch(&self, slots_per_epoch: u64) -> Option<Duration>;
 }

eth2/utils/slot_clock/src/system_time_slot_clock.rs

@@ -65,6 +65,35 @@ impl SlotClock for SystemTimeSlotClock {
         }
     }
 
+    fn duration_to_next_epoch(&self, slots_per_epoch: u64) -> Option<Duration> {
+        let now = SystemTime::now().duration_since(UNIX_EPOCH).ok()?;
+        let genesis = self.genesis_duration;
+
+        let slot_start = |slot: Slot| -> Duration {
+            let slot = slot.as_u64() as u32;
+            genesis + slot * self.slot_duration
+        };
+
+        let slot = self
+            .now()
+            .map(|slot| slot.epoch(slots_per_epoch))
+            .map(|epoch| (epoch + 1).start_slot(slots_per_epoch))?;
+
+        if now >= genesis {
+            Some(
+                slot_start(self.now()? + 1)
+                    .checked_sub(now)
+                    .expect("The next epoch cannot start before now"),
+            )
+        } else {
+            Some(
+                genesis
+                    .checked_sub(now)
+                    .expect("Control flow ensures genesis is greater than or equal to now"),
+            )
+        }
+    }
+
     fn slot_duration(&self) -> Duration {
         self.slot_duration
     }

eth2/utils/slot_clock/src/testing_slot_clock.rs

@@ -37,6 +37,11 @@ impl SlotClock for TestingSlotClock {
         Some(Duration::from_secs(1))
     }
 
+    /// Always returns a duration of `1 * slots_per_epoch` second.
+    fn duration_to_next_epoch(&self, slots_per_epoch: u64) -> Option<Duration> {
+        Some(Duration::from_secs(1 + slots_per_epoch))
+    }
+
     /// Always returns a slot duration of 0 seconds.
     fn slot_duration(&self) -> Duration {
         Duration::from_secs(0)