Implement extra pyfilesystem methods to improve efficiency

[rkhwaja]

• files_copy_v2 copies files much more efficiently than downloading all the data and uploading it again. copy
• files_create_folder_batch would improve the speed of makedirs
• files_delete would improve the speed of removetree and removedir
• files_move_v2 moves files much more efficiently than the default. Can also move directories
• walk maybe could use the recursive flag on files_list_folder but there's no override available anyway

[rkhwaja]

Reimplementing copydir

default implementation

• Default implementation contains 2 calls like exists, getinfo which could be amalgamated to one
• No option on dropbox call to preserve mtime but that might not be honored for all filesystems anyway
• Can't handle create=False case with Dropbox call - would have to precheck in order to follow the contract and to throw ResourceNotFound
• Can only precheck for DirectoryExpected, I think