fix(store): speed up Store.open by avoiding empty/clear calls unless needed

[jhamman]

We were calling store.empty and store.clear more than we needed to. This changes the open logic to only call clear when mode='w' and only call empty when mode='w-'.

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
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)