Need an undeprecated Reshuffle transform

`Reshuffle` transform is a convenient wrapper around GBK. It preserves user windowing correctly which is not always obvious for users. It usage has grown over many months since it was deprecated as it makes it easier for potential incorrect/unportable use of GroupByKey semantics. See [dev thread](https://lists.apache.org/thread.html/820064a81c86a6d44f21f0d6c68ea3f46cec151e5e1a0b52eeed3fbf@%3Cdev.beam.apache.org%3E) for more discussion.

There is a broad agreement we do need a transform that is meant for its common used purpose in user pipeline : to control parallelism (e.g. to control connections to a DB). 

Imported from Jira [BEAM-4372](https://issues.apache.org/jira/browse/BEAM-4372). Original Jira may contain additional context.
Reported by: rangadi.