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/sharded.rs

@@ -26,8 +26,8 @@ pub struct TableShardedReplication {
 
 impl TableReplication for TableShardedReplication {
 	fn read_nodes(&self, hash: &Hash) -> Vec<Uuid> {
-		let ring = self.system.ring.borrow().clone();
-		ring.walk_ring(&hash, self.replication_factor)
+		let ring = self.system.ring.borrow();
+		ring.get_nodes(&hash, self.replication_factor)
 	}
 	fn read_quorum(&self) -> usize {
 		self.read_quorum
@@ -35,7 +35,7 @@ impl TableReplication for TableShardedReplication {
 
 	fn write_nodes(&self, hash: &Hash) -> Vec<Uuid> {
 		let ring = self.system.ring.borrow();
-		ring.walk_ring(&hash, self.replication_factor)
+		ring.get_nodes(&hash, self.replication_factor)
 	}
 	fn write_quorum(&self) -> usize {
 		self.write_quorum