configuration message versioning

[Geal]

Configuration messages hold a version number, that should be used when parsing messages, since people might use an older tool with a new version of sozu.

The solution would be based on serde's tagged enum feature: serde-rs/serde#745 (a PR is currently in progress for this: serde-rs/serde#973 )

Here's how the code would change in command/src/data.rs:

first define a ConfigMessageWrapper enum:

#[derive(Debug,Clone,PartialEq,Eq,Hash,Serialize,Deserialize)]
#[serde(tag = "version")]
pub enum ConfigMessageWrapper {
  #[serde(rename = "0")]
  V0(ConfigMessageV0),
  #[serde(rename = "1")]
  V1(ConfigMessage),
}

The latest version is always the currently used ConfigMessage. ConfigMessageWrapperimplements Into<ConfigMessage>, so we need a way to transform a V0 message in a V1 one. It also has a function to get the version number.

Duplicate the assets in command/assets, so that we have subfolders for v0, v1, etc.

When the master parses messages, it will get the version number and hold onto it in the Client structure, to answer with the same version number.

Until the version 1.0 of sozu is released, the message format will have breaking changes, so for now the version number stays at 0.

[Geal]

closing as duplicate of #104