[#1725] Datastore must be the first IDatastore plugin loaded

This allows extensions to rely on it running before them.
[#1725] Add docstrings to the IDatastore methods
[#1725] Rename datastore_validate_query() to datastore_validate()

The previous name sounded like we were validating the `query_dict`, but we're
actually validating the `data_dict`. I felt `datastore_validate_data_dict()`
would be too big, so `datastore_validate()` seems better.