Allow p2p shuffle kwarg for DataFrame merges

[fjetter]

Symmetric change to dask/distributed#7514 that will use a specialized HashJoin layer.

Not sure how to test this, yet. Suggestions welcome

TODO:

• This is not respecting the dask.config("shuffle") value

[fjetter]

I'm using get_default_shuffle_algorithm now as well for the merge. with #9991 this would be enabled as well, be default

[fjetter]

Since this is a very low impact change that has only an effect if one provides the keyword p2p and this is blocking dask/distributed#7514 I will go ahead and merge this.

If there is anything wrong with it, I'll take care of it of course.