refactor: Use ReplicationFactor type in more places

- Remove the replication_factor.replication_factor() in favor of
  usize::from(replication_factor) to make the conversion more explicit.

- Implement Display on ReplicationFactor so that it can be formatted
  without converting to usize

- Use ReplicationFactor in the constructor of LayoutVersion and add a
  method to get a ReplicationFactor from a LayoutVersion, despite
  LayoutVersion still storing it as usize internally.

src/rpc/system_metrics.rs

@@ -68,7 +68,7 @@ impl SystemMetrics {
 				let replication_factor = system.replication_factor;
 				meter
 					.u64_value_observer("garage_replication_factor", move |observer| {
-						observer.observe(replication_factor.replication_factor() as u64, &[])
+						observer.observe(usize::from(replication_factor) as u64, &[])
 					})
 					.with_description("Garage replication factor setting")
 					.init()