DebuggerState refector

[xusheng6]

Refine the signatures of the interface functions, most notably the target control functions, e.g., SingleStep(). Also remember to define its semantic, i.e., whether it should return immediately, or it should return when the target stops for the first time.

[xusheng6]

This is mostly done. After various experiments, I have decided that a synchronous execution model is better suited for us. This differs from the decision made by VSCode or Visual studio. It is closer to that of windbg. I still make heavy use of debugger events to notify the change of the debugger status.

[xusheng6]

Also, the DebuggerState's role is downgraded to a collection of cached information of the debugger. The control related functionalities are moved into the DebuggerController class, which is the core of the debugger.