…-allocator

…-allocator

…idsai#1170)

Closes rapidsai#1169.

Essentially, we are running into the situation described in https://cython.readthedocs.io/en/latest/src/userguide/extension_types.html#disabling-cycle-breaking-tp-clear with `UpstreamResourceAdaptor`.

The solution is to prevent clearing of `UpstreamResourceAdaptor` objects by decorating them with `no_gc_clear`.

Cython calls out the following:

> If you use no_gc_clear, it is important that any given reference cycle contains at least one object without no_gc_clear. Otherwise, the cycle cannot be broken, which is a memory leak.

The other object in RMM that we mark `@no_gc_clear` is `DeviceBuffer`, and a `DeviceBuffer` can keep a reference to an `UpstreamResourceAdaptor`. But, an `UpstreamResourceAdaptor` cannot keep a reference to a `DeviceBuffer`, so instances of the two cannot form a reference cycle AFAICT.

Authors:
  - Ashwin Srinath (https://github.com/shwina)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Mark Harris (https://github.com/harrism)

URL: rapidsai#1170