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ARROW-13542: [C++][Compute][Dataset] Add dataset::WriteNode for writing rows from an ExecPlan to disk

This PR adds a write node.  The write node takes in `FileSystemDatasetWriteOptions` and the projected schema and writes the incoming data to disk.  It is a sink node but it is a bit different than the existing sink node.  The existing sink node transferred batches via an AsyncGenerator but that puts ownership of the batches outside of the push-based flow of the exec plan.  I added a new ConsumingSinkNode which consumes the batches as part of the push-based flow.  This makes it possible to block the exec plan from finishing until all data has been written to disk.

In addition this PR refines the AsyncTaskGroup a little.  `WaitForTasksToFinish` was not a very clearly named method.  Once called it actually transitioned the task group from a "top level tasks can be added" state to a "no more top level tasks can be added" state and that was not clear in the name.  The new name (`End`) is hopefully more clear.

This PR does not solve the backpressure problem.  Instead a serial async task group was created.  This will run all tasks in order (the default task group allows them to run in parallel) but not necessarily on the calling thread (i.e. unlike SerialTaskGroup we do not block on the AddTask method).  This allows tasks to pile up in a queue and, if the write is slow, this will become a pile-up point which will eventually run out of memory (provided there is enough data being written).  That problem is solved in a follow-up #11286

The `AsyncSerializedTaskGroup` and `AsyncTaskGroup` classes have very similar APIs but I did not create an interface / abstract base class because I don't yet envision any case where they would be interchangeable.  The distinction is a "can these tasks run in parallel or not" and is not a performance / resource question.

As a consequence of using the ExecPlan dataset writes no longer have reliable ordering.  If you pass in three batches and they are all destined for the same file then the batches may be written in any order in the destination file.  This is because the ExecPlan creates a thread task for each input batch and so they could arrive at the write node in any order.

Closes #11017 from westonpace/feature/ARROW-13542-write-node

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>