FileOperation: call close() explicitly in destructor

Assure the closing order of source and dest
FtpConnectionCache: add closeConnection()
Do not use ftp connection cache

Cache causes the following problem.

1. If using a separate FTP instances, file transfering in a same server
does not work due to too many instances

2. If reusing a cache connection in a transfer thread to avoid the above
problem before file transfering, aborting to download does not work

3. If closing a cache connection before filter transfering, after
aborting to transfer, listing a directory fails.