The API should provide a way to watch for changes.

There are various solutions to do that:

• A single event bus, similar to the engine (Events() RPC)
• A per-type watch (e.g. WatchTasks(), WatchServices(), ...)
• Blocking get with sequencer. (e.g. GetService() then GetService(IfModified: lastServiceVersion)

/cc @stevvooe @vieux