Is your feature request related to a problem? Please describe.
In order to maximize efficiency and utilization of the GPU, you need to be able to execute kernels and memory copies on independent streams. However, no libcudf API currently exposes streams to the end user.

Describe the solution you'd like
Any API that executes a GPU kernel or allocates/copies memory should provide the end user with the option to specify a stream.

Open questions:

• How should the stream be exposed? E.g., accept a cudaStream_t*? Or bundled inside of an options struct?
• Who is responsible for creating/destroying the stream?
• In a C++ API its easy to provide a default argument for the stream, but in the C API the user will always be required to specify a stream. Is this what we want?
• Should streams be exposed in all of the Python APIs?

I suspect we'll all agree this does need to be done, the question is rather one of how and when.