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ARROW-13650: [C++] Create dataset writer to encapsulate dataset writer logic

This ended up being a fairly comprehensive overhaul of the existing dataset writer mechanism.  ~~This PR relies on #10968 and will remain in draft until that is completed.~~

Breaking Changes:

 * The dataset writer no longer works with the synchronous scanner.  I don't think this would be a huge change but I think the current plan is to hopefully deprecate the synchronous scanner. (ARROW-13338)  This required changes in a python/r/ruby which will presumably be reverted when ARROW-13338 is done.
 * The default behavior is now to error if the output directory has any existing data.  This can be controlled with `existing_data_behavior` (see below)
 * Previously a single global counter was used for naming files.  This PR changes to a counter per directory.  So the following...
 ```
 /a1/b1/part-0.parquet
 /a1/b1/part-2.parquet
 /a1/b2/part-1.parquet
 ```
...would be impossible.  Instead you would receive...
```
/a1/b1/part-0.parquet
/a1/b1/part-1.parquet
/a1/b2/part-0.parquet
```
...this does not, however, mean that the resulting data files will be deterministic.  If the data in `/a1/b1/part-0.parquet` and `/a1/b1/part-1.parquet` originated from two different files being scanned in an unordered fashion then either part could represent either file.  A number of test cases in all implementations had to change as the expected paths for dataset writes changed.

* New features:
  * The dataset writer now works with the async scanner (ARROW-12803)
  * The dataset writer now respects backpressure (closes ARROW-2628?, related to but does not fully solve ARROW-13590 and ARROW-13611) and will stop pulling from the scanner when `max_rows_queued` (provided as an argument to `DatasetWriter`) is exceeded.  By default `max_rows_queued` is 64M.  This is not an "option" as it I don't think it should be exposed to the user.  I think it would be offering too many knobs.  I think eventually we may want to wrap up all backpressure into a single configurable setting.
  * `FileSystemDatasetWriteOptions` now has a `max_rows_per_file` setting (ARROW-10439).
  * `FileSystemDatasetWriteOptions` now has a `max_open_files` setting (ARROW-12321) which prevents opening too many files.  Instead the writer will apply backpressure on the scanner while also closing the open file with the greatest # of rows already written (then resume writing once the file is closed).
  * `FileSystemDatasetWriteOptions` now has a `existing_data_behavior` setting (ARROW-12358, ARROW-7706) which controls what to do if there is data in the destination.

 * Deferred for future work:
   * Add the new options to the python/R APIs (ARROW-13703)
   * Limiting based on file size (ARROW-10439)
   * More fine grained error control (ARROW-14175)

Closes #10955 from westonpace/feature/ARROW-13542--c-compute-dataset-add-dataset-writenode-for

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