Many improvements on ring/replication and its configuration:

- Explicit "replication_mode" configuration parameters that takes
  either "none", "2" or "3" as values, instead of letting user configure
  replication factor themselves. These are presets whose corresponding
  replication/quorum values can be found in replication/mode.rs

- Explicit support for single-node and two-node deployments
  (number of nodes must be at least "replication_mode", with "none"
  we can have only one node)

- Ring is now stored much more compactly with 256*8 + n*32 bytes,
  instead of 256*32 bytes

- Support for gateway-only nodes that do not store data
  (these nodes still need a metadata_directory to store the list
  of bucket and keys since those are stored on all nodes; it also
  technically needs a data_directory to start but it will stay
  empty unless we have bugs)

src/table/replication/mode.rs

@@ -0,0 +1,47 @@
+pub enum ReplicationMode {
+	None,
+	TwoWay,
+	ThreeWay,
+}
+
+impl ReplicationMode {
+	pub fn parse(v: &str) -> Option<Self> {
+		match v {
+			"none" | "1" => Some(Self::None),
+			"2" => Some(Self::TwoWay),
+			"3" => Some(Self::ThreeWay),
+			_ => None,
+		}
+	}
+
+	pub fn control_write_max_faults(&self) -> usize {
+		match self {
+			Self::None => 0,
+			_ => 1,
+		}
+	}
+
+	pub fn replication_factor(&self) -> usize {
+		match self {
+			Self::None => 1,
+			Self::TwoWay => 2,
+			Self::ThreeWay => 3,
+		}
+	}
+
+	pub fn read_quorum(&self) -> usize {
+		match self {
+			Self::None => 1,
+			Self::TwoWay => 1,
+			Self::ThreeWay => 2,
+		}
+	}
+
+	pub fn write_quorum(&self) -> usize {
+		match self {
+			Self::None => 1,
+			Self::TwoWay => 2,
+			Self::ThreeWay => 2,
+		}
+	}
+}