Unify the interface of GetTS and GetLocalTS

## Feature Request

### Describe your feature request related problem

Since the PD is able to allocate the Local TSO and Global TSO now, we add a new `GetLocalTS` method to get a TSO from the specific `dc-location`. However, you can also get a Global TSO with `GetLocalTS` by passing the `"global"` as the `dc-location` parameter, which is equivalent to the `GetTS` method.

### Describe the feature you'd like

I think we should provide a new method to unify the interface of `GetTS` and `GetLocalTS`, rather than providing two different methods.