Fix copy and sync bugs. Remove extra sync in default stream.

- NULL stream was waiting for itself to be empty before each command.
- Force "blocking" streams to wait for NULL to empty.  This was missing
  before.
- async copy was disabling itself via trueAsync=false for common cases.

Refactor:
- rename _null_stream to _default_stream.
- move some null sync function to defaultSync, move to dev member func.

src/staging_buffer.cpp

@@ -4,14 +4,11 @@
 
 #include "hcc_detail/staging_buffer.h"
 
-#ifndef tprintf
-#define tprintf(trace_level, ...) 
-#endif
-
 #ifdef HIP_HCC
 #define THROW_ERROR(e) throw ihipException(e)
 #else
 #define THROW_ERROR(e) throw 
+#define tprintf(trace_level, ...) 
 #endif
 
 //-------------------------------------------------------------------------------------------------