FTAM — ftam/ftp issue: lifecycle of FileTransferSession, Directory, and File

Once a FileTransferSession object is created on behalf of a client a
large number of Directory and File objects can be created by calls to
list and get_file. The specification does not address reclaiming
server side resources when these objects are no longer required by the
client.

Once FileTransferSession::logout is called it can be inferred that all
of the associated Directory and File objects should no longer be valid
and can be reaped. The specification should state that they must no
longer be valid, as you should not have access to a file or directory
after you have logged out.

If after using a number of file and directories a client has no
further use for them, there should be a way for the client to inform
the server. This requires a destroy() method on the File interface.

Also, a robust virtual file system implementation must be allowed to
reap FileTransferSession, Directory, and File objects at any time in
order to reclaim resources from ill-behaved clients that acquire but
do not release objects. The spec should mention that all clients, but
particularly those that don't actively use File and Directory
references for long periods, can expect OBJECT_NOT_EXIST exceptions.

see above