Delete cuda_async_memory_resource copy/move ctors/operators (#860)

Fixes #859

Deletes the copy/move ctors to avoid erroneous sharing of the underlying cudaMemPool_t handle.

Authors:
  - Jake Hemstad (https://github.com/jrhemstad)

Approvers:
  - Rong Ou (https://github.com/rongou)
  - Mark Harris (https://github.com/harrism)

URL: #860

include/rmm/mr/device/cuda_async_memory_resource.hpp

@@ -108,10 +108,10 @@ class cuda_async_memory_resource final : public device_memory_resource {
     RMM_ASSERT_CUDA_SUCCESS(cudaMemPoolDestroy(pool_handle()));
 #endif
   }
-  cuda_async_memory_resource(cuda_async_memory_resource const&) = default;
-  cuda_async_memory_resource(cuda_async_memory_resource&&)      = default;
-  cuda_async_memory_resource& operator=(cuda_async_memory_resource const&) = default;
-  cuda_async_memory_resource& operator=(cuda_async_memory_resource&&) = default;
+  cuda_async_memory_resource(cuda_async_memory_resource const&) = delete;
+  cuda_async_memory_resource(cuda_async_memory_resource&&)      = delete;
+  cuda_async_memory_resource& operator=(cuda_async_memory_resource const&) = delete;
+  cuda_async_memory_resource& operator=(cuda_async_memory_resource&&) = delete;
 
   /**
    * @brief Query whether the resource supports use of non-null CUDA streams for