Any backup & restore operation may be leaving opened files
if a file logger is being used. This implementes the close()
method, so every logger can close any resource.

Also, the recommended backup_controlled::destroy() method
now calls to new logger::destroy() method in charge of
deleting all the references and closing any resource.

Finally, some internally used controllers, were missing
their destroy call, leading to associated loggers to
remain open. Now all them are explicitly deltroyed.