Support callback for Matrix Client request

[alanpoon]

This makes accessing Matrix Client in background thread and callback in UI thread easier.

This follows the design of cx.http_request.

[kevinaboos]

Thanks Alan, I like the general idea of abstracting generic functions to perform arbitrary backend functions in background tasks.

I do have a few questions:

1. My main concern with this design pattern is that it seems to encourage usage of the Client object. In most cases, accessing a piece of data through the Client is the least efficient way to do so, so I'd like to hear about more use cases that motivate this.
2. why does the request_id need to be a LiveId? Live IDs are typically related to a UI element or widget or something?
   • Seems like it could be any generic type that implements PartialEq.
3. the func closure returns a Boxed Any. Is that necessary? Any has quite a few limitations; I'm wondering if we can also use generics here, but we don't necessarily have to do that at first.
   • In fact, you already use a generic type T within the MatrixClientAction enum...