In Interpreter, allow a node to have more than one consumer

In Interpreter, allow a node to have more than one consumer. In other words, the graph may be a DAG not just a tree.

If a node has more than one consumer, its Sink is a DuplicatingSink that writes to multiple consumers. Each has an independent ArrayDeque.

This change will only be exercised when CALCITE-2116 is fixed, and the planner starts producing DAG plans.