Added Comm close handler for the data channel of FtpStateData

transaction in preparation for officially dropping connect callbacks for
closing descriptors.

The data channel can be opened and closed a few times and the descriptor
must be kept in sync with the close handler. I factored out the
open/closing code into a simple FtpChannel class. That class is now used
for both FTP control and data channels.

The changes resolve one XXX discussion regarding FTP not having a close
handler for the data channel. On the other hand, adding a second close
handler attached to the same transaction is not a trivial change as the
side-effects of Squid cleanup code are often illusive.

For example, I suspect that FTP cleanup code does not close or even
check the control channel. I added a DBG_IMPORTANT statement to test
whether the control channel remains open. Or should that be an assert()?

I think that only one out of the two callbacks can be dialed because the
close handler executed first will invalidate the transaction object.