[RFC] Higher-level AdapterManager

[Yoric]

Spinoff from an idea by @mcav in #53.

We could rewrite the AdapterManager's channel API as follows:

/// A channel, which may have read/write capabilities.
pub struct Channel {
  id: Id<Channel>,
  kind: ChannelKind,
  service: Id<ServiceId>,
  pub properties: PropertyBag,
  pub fetch: Option<Box< Fn(FetchArgs) -> Result<Value, Error> + Sync + Send>>,
  pub send: Option<Box< Fn(SendArgs) -> Result<(), Error> + Sync + Send>>,
  pub watch: Option<Box< Fn(WatchArgs) -> Result<WatchGuard, Error> + Sync + Send>>,
  // Possibly more in the future.
}
impl Channel {
  /// Create an empty channel.
  pub fn new(id: &Id<Channel>, service: &Id<ServiceId>, kind: ChannelKind) -> Self;
}

impl AdapterManger {
  pub fn add_channel(channel: Channel) -> Result<(), Error>
}

Pros

• This relieves Adapters from having to maintain their own mapping between Id<Getter>/Id<Setter>, actual devices, actual implementation.
• Extending this with future capabilities (e.g. delete) should be quite simple.
• We can expose a data structure Channel much like this to the REST API, and it should often be easier to use than our current separate getters/setters.

Cons

• The current AdapterManager manages to combine various fetch (respectively send, watch) requests by Adapter and send them together to minimize communications. I don't see how to do this with this design.

[Yoric]

Cc @fabricedesre , @cr , @julienw , @dhylands , @azasypkin , @aosmond .