btr support for para_client.

btr support for para_client.

The use of the client code in audiod requires a node to be inserted
as the _parent_ of some existing node, which was not neccessary
before. Therefore a new btrn pointer for the child node is added to
struct btr_node_description and btr_new_node() is adjusted accordingly.

The patch also adds a small helper sched_request_barrier() to
the scheduler.