Add a test case testing memory stability in the presence of multiple sub-dataset load operations

[sjperkins]

Thanks for contributing to xarray-ms.

We would appreciate it if you could add:

• Test Cases covering your PR.
• Documentation.
• A Changelog entry in doc/source/changelog.rst.

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📚 Documentation preview 📚: https://xarray-ms--166.org.readthedocs.build/en/166/

[sjperkins]

@landmanbester Can you confirm that the added test case reflects the type of usage patterns in pfb?

Memory usage stays flat on an actual MS. Perhaps there's some interaction with ray that we need to figure out.

[landmanbester]

Yeah, that does look correct. It's probably the Ray interaction that's causing it (maybe references get stuck in the plasma store?). I'll try to make a simple reproducer. From what I recall, xarray-kat did not have this problem in case that tells you anything