Duracloud: Handle fetching many files from a space

Duracloud returns a maximum of 1000 files when browsing a space. Update code to call that endpoint repeatedly to fetch all files with a given prefix. To handle the larger number of files, convert _get_files_list to an
iterator.  Update browse to deal with paths one at a time, instead of fetching all of them at once.

Update tests for new calls.  Split some tests into multiple for ease of testing and changing later (eg test_move_to_storage_service -> test_move_to_storage_service_file & test_move_to_storage_service_folder

refs #7481