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/network/src/attestation_service/mod.rs

@@ -32,7 +32,7 @@ const LAST_SEEN_VALIDATOR_TIMEOUT: u32 = 150;
 // 30 mins at a 12s slot time
 /// The fraction of a slot that we subscribe to a subnet before the required slot.
 ///
-/// Note: The time is calculated as `time = milliseconds_per_slot / ADVANCE_SUBSCRIPTION_TIME`.
+/// Note: The time is calculated as `time = seconds_per_slot / ADVANCE_SUBSCRIPTION_TIME`.
 const ADVANCE_SUBSCRIBE_TIME: u32 = 3;
 /// The default number of slots before items in hash delay sets used by this class should expire.
 ///  36s at 12s slot time

beacon_node/network/src/service.rs

@@ -269,7 +269,7 @@ fn spawn_service<T: BeaconChainTypes>(
                 _ = service.metrics_update.next() => {
                     // update various network metrics
                     metric_update_counter +=1;
-                    if metric_update_counter* 1000 % T::EthSpec::default_spec().milliseconds_per_slot == 0 {
+                    if metric_update_counter % T::EthSpec::default_spec().seconds_per_slot == 0 {
                         // if a slot has occurred, reset the metrics
                         let _ = metrics::ATTESTATIONS_PUBLISHED_PER_SUBNET_PER_SLOT
                             .as_ref()