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ARROW-12584: [C++][Python] Expose method for benchmarking tools to release unused memory from the allocators

@pitrou What are your thoughts on this.  It's not urgent (benchmarking can always use the system allocator in the meantime if they are having RAM issues and I think they reduced already which benchmarks they run on the RAM-limited servers)

With mimalloc the following command has a max RSS of ~16GB.
```
conbench file-read fanniemae_2016Q4 --all=true
```

If I add a call to release_unused between runs then the max RSS drops to ~10GB.

Closes #10186 from westonpace/feature/ARROW-12584--c-python-expose-method-for-benchmarking-tools

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>