Make closing of `Downloader`s more concise.
*NOTE: This is only a WIP branch in so far as I haven't tested it, as it was done on the train before my holiday. The code is final though, so if people are happy it works, please merge.*
The base `Downloader` class now wraps the `InputStream` returned by
any child classes, in order to notify the child class when that stream
is closed. This prevents each child class having to figure out a way
to be notified of this.
This helps keep the API concise, because close handling is dealt with
without the need to add any public methods to the `Downloader` class
hierarchy.
Also removed some dead code which was unused.
See merge request !139
47196234fe