Use HighLevelGraph layers in dask-mongo.read_mongo

[jrbourbeau]

As a follow-up to #2, we should update dask-mongo.read_mongo to use HighLevelGraph layers (specifically DataFrameIOLayer). This should look very similar to what's done in dask-snowflake.

cc @ncclementi

[crusaderky]

To clarify: Bag.dask is already a HighLevelGraph. This issue is about replacing the basic MaterializedLayer, which is automatically generated by the Bag constructor and contains 1 key per partition, to a bag-specific I/O class.
Since no such class exists today (correct me if I'm wrong), it would need to be first implemented in dask.bag and then used here.

[ncclementi]

@crusaderky I believe you are correct, this was open by James when we were implementing things to work with writing and reading into dataframes. @jrbourbeau Should we close this issue?

[crusaderky]

@ncclementi it's still valid - it's just missing the prerequisite infrastructure in the dask package.

[jrbourbeau]

Yeah, let's keep it open, but know that it is low priority