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ARROW-4913:[Java][Memory] Add additional methods for observing allocations.

- Additional methods for observers to calculate total number of live ledgers and buffers.
- Allows clients to clamp down on heap growth due to these buffers.

Author: Praveen <praveen@dremio.com>
Author: Jacques Nadeau <jacques@apache.org>

Closes #4012 from praveenbingo/ARROW-4913 and squashes the following commits:

4671d032 <Praveen> Fix some checkstyle failures.
5af55057 <Jacques Nadeau> ARROW-TBD: Add additional methods to the allocation listener interface for PreAllocation and Release.