Refactor the code to make it easy to add plugins

[doitian]

• Re-organize the code so it's easy to implement a plugin in its own crate. Take care of the dependencies between crates. Also create a template for the pool extension crate. Now adding a plugin requires a lot of boilerplate code.

• Make it easy to add plugins in Rust. For example, add an Service struct, that we can use it to start the service in our own src/main.rs:

  import otx_service;
  
  fn main() {
      let mut service = OtxService::new();
      service.add_plugin(MyPlugin::new());
      
      service.run();
  }