[FEA] Improvements to RMM stream pool

[harrism]

Is your feature request related to a problem? Please describe.
Extend RMM stream_pool with the same features as were recently added to libcudf in rapidsai/cudf#13922

• add a base stream_pool interface class
• implement the interface using a pool like the current one
• Also implement a default_stream_pool that always returns the default stream (for debugging)
• Add fork / join functions for forking a set of streams from a specified stream (using the pool to supply streams), and to join a list of streams to another stream.

Describe the solution you'd like
rapidsai/cudf#13922