Drivers' Data Service Helper

Create a helper in order to allow drivers to send requests to the Data Service.

This helper is a class that encapsulates code from drivers, and provide methods to common Data Service operations, like data copy.

For Data Copy, the following workflow has been designed:

Driver calls helper method copy_share_data, passing as arguments: context, source_share_id, source_mount_command, source_path, destination_share_id, destination_mount_command, destination_path and callback_function_name.

These arguments will be passed to the Data Service, which will allow access to the respective shares, execute the mount command, copy data according to the paths provided, unmount, and deny access.

The Data Service will issue 2 RPC calls to the hosts of the involved shares, passing the share_id, the callback function name and an error constant. The manager will receive the RPC calls, if the error constant is not None, it will throw an error, else, it will call the callback function name which should be implement in the driver, to run any code after copying has completed.

The driver can obtain a reference to the data service helper through kwards in the constructor, similar to how it is done today with private storage.