Make explicit links between elements that are used to activate the
next element in the graph.
Make subgraphs a special regular node. Make a link from the
subgraph children to the parent so that the subgraph completes when
all the children completed.
Implement a single process function in plugins
Remove many messages in the client node
Don't unlink the ports of an inactive link because then it might be
possible that the scheduler can't recycle a buffer, instead use
a port flag to mark the ports disabled and change to scheduler to
skip those ports.
