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/mod.rs

@@ -1,8 +1,10 @@
 mod parameters;
 
 mod fullcopy;
+mod mode;
 mod sharded;
 
 pub use fullcopy::TableFullReplication;
+pub use mode::ReplicationMode;
 pub use parameters::*;
 pub use sharded::TableShardedReplication;