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ARROW-4120: [Python] Testing utility for checking for "macro" memory leaks detectible with psutil.Process

Also adds a unit test for ARROW-7956 that fails in 0.15.0 and 0.15.1 but passes in 0.16.0.

Since these tests are time consuming I set them off by default, but we need to run them regularly. I opened ARROW-8048 about setting up a nightly build to run these.

Closes #6551 from wesm/ARROW-4120 and squashes the following commits:

94e077f30 <Wes McKinney> Allow for pandas-free build
2c28ccbff <Wes McKinney> Fix non-idiomatic code
e9e1451f7 <Wes McKinney> Remove psutil since optional
abf9997ed <Wes McKinney> docstring typo
9ae59b9c1 <Wes McKinney> Add utility for testing for macro memory use leaks non detectible through Arrow memory pools. Tests for ARROW-7956
a86342530 <Wes McKinney> Add utility to help find "macro" memory leaks that are undetectable through Arrow memory pools

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>