FB 32172 introduce deleteDatastoreNamed method 

Method and test cases.

This will delete the datastore and all associated files, including attachments and extensions.

Also disallow datastores to have the suffix _extensions, to avoid confusion with the
${datastore}_extensions directory.
At this point we're testing that the directories created are what we expect them to be called. Using `-attachmentStorePath` would mean we'd not be validating our understanding of where the paths are going is correct.