test: check that store, array, and group classes are serializable

[jhamman]

This PR adds tests to insure that our store/array/group classes are serializable w/ using pickle and can be reliably roundtripped. This is important for Dask and other libraries that need to be able to share Zarr objects between processes/machines.

A necessary side effect of this work was the creation of a few additional __eq__ methods.

closes #1760

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [x] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)