Represent slots in secs instead of millisecs (#2163)

## Issue Addressed

NA

## Proposed Changes

Copied from #2083, changes the config milliseconds_per_slot to seconds_per_slot to avoid errors when slot duration is not a multiple of a second. To avoid deserializing old serialized data (with milliseconds instead of seconds) the Serialize and Deserialize derive got removed from the Spec struct (isn't currently used anyway).

This PR replaces #2083 for the purpose of fixing a merge conflict without requiring the input of @blacktemplar.

## Additional Info

NA


Co-authored-by: blacktemplar <blacktemplar@a1.net>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -639,7 +639,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
                 let start_slot = head_state.slot;
                 let task_start = Instant::now();
-                let max_task_runtime = Duration::from_millis(self.spec.milliseconds_per_slot);
+                let max_task_runtime = Duration::from_secs(self.spec.seconds_per_slot);
 
                 let head_state_slot = head_state.slot;
                 let mut state = head_state;

beacon_node/beacon_chain/src/eth1_chain.rs

@@ -90,7 +90,7 @@ fn get_sync_status<T: EthSpec>(
 
         let period_start = slot_start_seconds::<T>(
             genesis_time,
-            spec.milliseconds_per_slot,
+            spec.seconds_per_slot,
             voting_period_start_slot,
         );
 
@@ -98,7 +98,7 @@ fn get_sync_status<T: EthSpec>(
     } else {
         // The number of seconds in an eth1 voting period.
         let voting_period_duration =
-            T::slots_per_eth1_voting_period() as u64 * (spec.milliseconds_per_slot / 1_000);
+            T::slots_per_eth1_voting_period() as u64 * spec.seconds_per_slot;
 
         let now = SystemTime::now().duration_since(UNIX_EPOCH).ok()?.as_secs();
 
@@ -454,7 +454,7 @@ impl<T: EthSpec> Eth1ChainBackend<T> for CachingEth1Backend<T> {
         let voting_period_start_slot = (state.slot / period) * period;
         let voting_period_start_seconds = slot_start_seconds::<T>(
             state.genesis_time,
-            spec.milliseconds_per_slot,
+            spec.seconds_per_slot,
             voting_period_start_slot,
         );
 
@@ -645,10 +645,10 @@ fn int_to_bytes32(int: u64) -> Vec<u8> {
 /// Returns the unix-epoch seconds at the start of the given `slot`.
 fn slot_start_seconds<T: EthSpec>(
     genesis_unix_seconds: u64,
-    milliseconds_per_slot: u64,
+    seconds_per_slot: u64,
     slot: Slot,
 ) -> u64 {
-    genesis_unix_seconds + slot.as_u64() * milliseconds_per_slot / 1_000
+    genesis_unix_seconds + slot.as_u64() * seconds_per_slot
 }
 
 /// Returns a boolean denoting if a given `Eth1Block` is a candidate for `Eth1Data` calculation
@@ -686,7 +686,7 @@ mod test {
         let voting_period_start_slot = (state.slot / period) * period;
         slot_start_seconds::<E>(
             state.genesis_time,
-            spec.milliseconds_per_slot,
+            spec.seconds_per_slot,
             voting_period_start_slot,
         )
     }
@@ -696,21 +696,21 @@ mod test {
         let zero_sec = 0;
         assert_eq!(slot_start_seconds::<E>(100, zero_sec, Slot::new(2)), 100);
 
-        let half_sec = 500;
-        assert_eq!(slot_start_seconds::<E>(100, half_sec, Slot::new(0)), 100);
-        assert_eq!(slot_start_seconds::<E>(100, half_sec, Slot::new(1)), 100);
-        assert_eq!(slot_start_seconds::<E>(100, half_sec, Slot::new(2)), 101);
-        assert_eq!(slot_start_seconds::<E>(100, half_sec, Slot::new(3)), 101);
-
-        let one_sec = 1_000;
+        let one_sec = 1;
         assert_eq!(slot_start_seconds::<E>(100, one_sec, Slot::new(0)), 100);
         assert_eq!(slot_start_seconds::<E>(100, one_sec, Slot::new(1)), 101);
         assert_eq!(slot_start_seconds::<E>(100, one_sec, Slot::new(2)), 102);
 
-        let three_sec = 3_000;
+        let three_sec = 3;
         assert_eq!(slot_start_seconds::<E>(100, three_sec, Slot::new(0)), 100);
         assert_eq!(slot_start_seconds::<E>(100, three_sec, Slot::new(1)), 103);
         assert_eq!(slot_start_seconds::<E>(100, three_sec, Slot::new(2)), 106);
+
+        let five_sec = 5;
+        assert_eq!(slot_start_seconds::<E>(100, five_sec, Slot::new(0)), 100);
+        assert_eq!(slot_start_seconds::<E>(100, five_sec, Slot::new(1)), 105);
+        assert_eq!(slot_start_seconds::<E>(100, five_sec, Slot::new(2)), 110);
+        assert_eq!(slot_start_seconds::<E>(100, five_sec, Slot::new(3)), 115);
     }
 
     fn get_eth1_block(timestamp: u64, number: u64) -> Eth1Block {