std::swap for nonblocking vectors is broken

[anyzelman]

Consider test 6 in tests/unit/dot.cpp:

	rc = grb::set( y, x );
	rc = rc ? rc : grb::set< grb::descriptors::use_index >( x, 0 );

where y on entry equals (0,1,...,n-1), which is also what the above code sets x to while y retains the old values of x.

In other words, the shorter form of the above is std::swap( x, y ), but replacing the above with that raises derails the nonblocking backend. At present, this is the smallest minimal example of the swap failing, though hopefully there are more minimalistic instances to be found to aid debugging.