Drop support for multiple Streams per AsdfFile

[braingram]

It appears asdf support multiple Stream objects (that share the same streamed block) in one AsdfFile. The usefulness of this is questionable and complicates streamed block handling. Support for this undocumented feature should be removed:

asdf/asdf/_tests/test_stream.py

Lines 50 to 69 in 3d46a99

	def test_stream_twice():
	"""
	Test that if you write nothing, you get a zero-length array
	"""
	buff = io.BytesIO()
	tree = {"stream": stream.Stream([6, 2], np.uint8), "stream2": stream.Stream([12, 2], np.uint8)}
	ff = asdf.AsdfFile(tree)
	ff.write_to(buff)
	for i in range(100):
	buff.write(np.array([i] * 12, np.uint8).tobytes())
	buff.seek(0)
	ff = asdf.open(buff)
	assert len(ff._blocks) == 1
	assert ff.tree["stream"].shape == (100, 6, 2)
	assert ff.tree["stream2"].shape == (50, 12, 2)