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ARROW-16676: [C++] ReservationListenableMemoryPool::Impl::bytes_allocated() should return its own number of bytes rather than the underlying pool's

ReservationListenableMemoryPool is a decorator of the underlying memory pool with its own mem counter. It should represent for the bytes reserved from itself rather than the underlying pool.

E.g. a ReservationListenableMemoryPool `pool-a`  instance wraps the default pool, and default pool's allocation is now 1GB, `pool-a`'s allocation is 10MB, we should let `pool-a.bytes_allocated()` return 10MB rather than 1GB.

It's not really a bug but can limit the information user can get when calling these APIs.

Closes #13251 from zhztheplayer/ARROW-16676

Lead-authored-by: Hongze Zhang <hongze.zhang@intel.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>