http.client: Better docstring.
ftp.command: Split read_stream into begin_stream & read_stream
Rewrite ftp.client interface to align with http.client interface

ftp.client: fetch() pairs with read_contnet(), fetch_file_listing()
pairs with read_listing_content()

processor.ftp: Use of the callback function is eliminated
ftp.command: Accept FTP code 125 on data connection