consensus-slots: cleanup SlotDuration config (#10878)

* consensus-slots: cleanup the SlotDuration config

* fix tests

* address review comments

substrate/primitives/timestamp/src/lib.rs

@@ -42,10 +42,16 @@ impl Timestamp {
 	}
 
 	/// Returns `self` as [`Duration`].
-	pub fn as_duration(self) -> Duration {
+	pub const fn as_duration(self) -> Duration {
 		Duration::from_millis(self.0)
 	}
 
+	/// Returns `self` as a `u64` representing the elapsed time since the UNIX_EPOCH in
+	/// milliseconds.
+	pub const fn as_millis(&self) -> u64 {
+		self.0
+	}
+
 	/// Checked subtraction that returns `None` on an underflow.
 	pub fn checked_sub(self, other: Self) -> Option<Self> {
 		self.0.checked_sub(other.0).map(Self)