[FEA] Draw a worker stream from a parent handle and set it as the user stream​ in a returned handle

[afender]

Is your feature request related to a problem? Please describe.
Many functions (including all graph prims) accept a handle and execute on the handle's user stream (accessed by handle_t.get_stream()​). This is an issue if the caller is also provided with a single handle but wants to run these functions concurrently. We need to leverage the pool of worker streams of the parent handle without having to change the signature and internals of low-level functions.

Describe the solution you'd like
One way is to add a raft::handle_t get_handle_from_pool feature that draws a worker stream from the parent handle and set it as the user stream​ of the returned handle.

Describe alternatives you've considered
Pass streams explicitly or rewrite prims to access worker streams from the handle

Additional context
Needed for rapidsai/cugraph#957