pithos: Bulk operation refactoring

The implementation for bulk deleting/copying/moving paths starting with the provided path + the delimiter argument (if such is provided) used to query the database for the specific paths and perform in a loop the same actions performed for the single delete/copy/move operations.
This commit substitutes the code inside the loop by calling the respective method for the derived path (using recursion).
Details for the relative API calls can be found here:
https://www.synnefo.org/docs/synnefo/latest/object-api-guide.html#delete
https://www.synnefo.org/docs/synnefo/latest/object-api-guide.html#id7
https://www.synnefo.org/docs/synnefo/latest/object-api-guide.html#id9