Introduce deleteDatastoreNamed method

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.
Merge pull request #37 from cloudant/32172-delete-datastore

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.