fix shutdown paths

this includes:
- move the gdb ownership back to the engine (thus strip down the
  adaptors as far as possible)
- make gdb startup synchronous
- make adapter shutdown synchronous
- fix the state transitions relating to shutdown

src/plugins/debugger/debuggerconstants.h

@@ -79,15 +79,13 @@ enum DebuggerState
 
     InferiorStopping,           // Debuggee running, stop requested
     InferiorStopped,            // Debuggee stopped
-    InferiorStopFailed,         // Debuggee stopped
+    InferiorStopFailed,         // Debuggee not stopped, will kill debugger
 
     InferiorShuttingDown,
     InferiorShutDown,
     InferiorShutdownFailed,
 
-    AdapterShuttingDown,
-    //AdapterShutDown,          // Use DebuggerNotReady instead
-    AdapterShutdownFailed,
+    EngineShuttingDown
 };
 
 enum DebuggerStartMode