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)

script/dev-cluster.sh

@@ -31,9 +31,7 @@ bootstrap_peers = [
   "127.0.0.1:3903"
 ]
 max_concurrent_rpc_requests = 12
-data_replication_factor = 3
-meta_replication_factor = 3
-meta_epidemic_fanout = 3
+replication_mode = "3"
 
 [s3_api]
 api_bind_addr = "0.0.0.0:$((3910+$count))"	# the S3 API port, HTTP without TLS. Add a reverse proxy for the TLS part.