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/replication_mode.rs

@@ -38,14 +38,10 @@ impl ReplicationFactor {
 		}
 	}
 
-	pub fn replication_factor(&self) -> usize {
-		self.0
-	}
-
 	pub fn read_quorum(&self, consistency_mode: ConsistencyMode) -> usize {
 		match consistency_mode {
 			ConsistencyMode::Dangerous | ConsistencyMode::Degraded => 1,
-			ConsistencyMode::Consistent => self.replication_factor().div_ceil(2),
+			ConsistencyMode::Consistent => usize::from(*self).div_ceil(2),
 		}
 	}
 
@@ -53,7 +49,7 @@ impl ReplicationFactor {
 		match consistency_mode {
 			ConsistencyMode::Dangerous => 1,
 			ConsistencyMode::Degraded | ConsistencyMode::Consistent => {
-				(self.replication_factor() + 1) - self.read_quorum(ConsistencyMode::Consistent)
+				(usize::from(*self) + 1) - self.read_quorum(ConsistencyMode::Consistent)
 			}
 		}
 	}
@@ -65,6 +61,12 @@ impl std::convert::From<ReplicationFactor> for usize {
 	}
 }
 
+impl std::fmt::Display for ReplicationFactor {
+	fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+		self.0.fmt(f)
+	}
+}
+
 pub fn parse_replication_mode(
 	config: &Config,
 ) -> Result<(ReplicationFactor, ConsistencyMode), Error> {