[C++] Add generic multi-output node

A node with multiple outputs is a bit of a tricky thing to create and deserving of its own node type. This node would be a pipeline breaker. It would need to schedule N tasks for each incoming tasks (potentially N-1 if you want to run one of the outputs on the current task). It will need to be clever about how it applies backpressure. If either downstream node asks for backpressure then the signal should be forwarded.

Another bonus is that if we can encapsulate this behavior in a generic node then we can get rid of the TeeNode because this would become a multi-output node where one output is a write node and the other output is the regular path.

**Reporter**: [Weston Pace](https://issues.apache.org/jira/browse/ARROW-16524) / @westonpace

_{**Note**: *This issue was originally created as [ARROW-16524](https://issues.apache.org/jira/browse/ARROW-16524). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by reviewing the existing TeeNode and the pipeline task-scheduling and backpressure behavior described in the issue. Define how a generic multi-output node schedules N tasks per incoming task, handles the current-task optimization, and forwards downstream backpressure; completion should support replacing TeeNode.

Written by the indexing model from the issue text.