Optimize the design of Router

[flycash]

Now the Router has two key interfaces:

However, we use Chain interface in our codes as the entrance, like:

We should notice that, the Router is the top abstraction. Using Chain in codes will reduce the expandability.

So I proposal to refactor the design.

Firstly, we keep the Router interface, but remove the Priority method;
Secondly, we keep Chain interface, but it will implement Router interface, which means that Chain is a Router too but it's more complicated.
Thirdly, we define PriorityRouter which is the Router interface + Priority method;
Finally, we will replace Chain with Router in our codes. The Router instance should be fetched from extension;

The class diagram is:

The benefit is that:

1. User can schedule many routers by their own way, not just Chain;
2. Only Router interface are exposed outside the Router module;

[zouyx]

i will check it later