Add refcount on MemObj in blocked scenarios.

- Prevents destruction of MemObj while it may still be in use.
- Add UNRECOVERABLE to check whether object is deleted while having
dependencies, fix all problems is tests due to that fact.
- Fix special queue setting, clean interfaces.

Change-Id: I2a467e80df00ea1650decdcfa6866acf10b441f8

unit_tests/scenarios/blocked_enqueue_with_callback_scenario_tests.cpp

@@ -25,6 +25,8 @@
 #include "unit_tests/mocks/mock_command_queue.h"
 #include "unit_tests/mocks/mock_kernel.h"
 #include "runtime/helpers/options.h"
+#include "runtime/platform/platform.h"
+#include "runtime/event/async_events_handler.h"
 
 #include "gtest/gtest.h"
 #include "test.h"
@@ -143,4 +145,6 @@ TEST_F(ScenarioTest, givenAsyncHandlerEnabledAndUserEventBlockingEnqueueAndOutpu
 
     clReleaseEvent(eventOut);
     clReleaseCommandQueue(clCommandQ);
+
+    platform()->getAsyncEventsHandler()->closeThread();
 }