stream: Make the topology a reference counted object.

The stream topology has no lock of its own resulting in
another lock protecting it in some way (for example the
channel lock). If multiple channels are being juggled at
the same time this can be problematic. This change makes
the topology a reference counted object instead which
guarantees it will remain valid even without the channel
lock being held.

Change-Id: I4f4d3dd856a033ed55fe218c3a4fab364afedb03