[BUG] Pool allocators are incompatible with compute-sanitizer memchecks

[bdice]

Is your feature request related to a problem? Please describe.
It is not possible to use compute-sanitizer memchecks with the RMM pool allocator. Because the pool is one large allocation, out-of-bounds writes with respect to suballocations are not caught by compute-sanitizer.

Describe the solution you'd like
First, we should document that the pool allocator does not work correctly when running compute sanitizer.

Second, NVTX has APIs that allow compute-sanitizer to track suballocations, and RMM should use them. https://docs.nvidia.com/compute-sanitizer/SanitizerNvtxGuide/index.html

cc: @jeaton32 @cjnolet @wence- @jrhemstad

[harrism]

For historical context: #790