## What changes were proposed in this pull request?

Move the internals of the PySpark accumulator API from the old deprecated API on top of the new accumulator API.

## How was this patch tested?

The existing PySpark accumulator tests (both unit tests and doc tests at the start of accumulator.py).

Author: Holden Karau <holden@us.ibm.com>

Closes #14467 from holdenk/SPARK-16861-refactor-pyspark-accumulator-api.