Debugger [CDB]: Release COM interfaces on RunControl finished.

Ensure that at most one instance of the CDBEngine is alive.
Prevent inconsistent debugger states if several sessions are started.
Squashed cherry-picked commits from master correcting the
plugin state handling (2efdeb5cd0,
57f7616286).

Reviewed-by: hjk
Task-number: QTCREATORBUG-2894

src/plugins/debugger/debuggerplugin.cpp

@@ -172,6 +172,13 @@
 // Transitions marked by '*' are done asynchronously.
 // The GdbEngine->setupEngine() function is described in more detail below.
 //
+// The engines are responsible for local roll-back to the last
+// acknowledged state before calling notify*Failed. I.e. before calling
+// notifyEngineSetupFailed() any process started during setupEngine()
+// so far must be terminated.
+//
+//
+//
 //                        DebuggerNotReady
 //                               +
 //                      EngineSetupRequested