Kobo Touch Extended holds disk space after transfer to device is complete

[bgalbrecht]

I'm running Calibre 2.70 with Kobo Touch Extended 2.8.2 on Kubuntu 16.04.4, and I was copying about 28 GB of epubs to my new Kobo Glo HD, in groups of about 5,000 books at a time, without shutting down Calibre in between. It was consuming about 65GB, presumably in temp files for the conversion of kepubs, and didn't release any of the space until I shut down Calibre. I admit, I don't expect to be copying this much to my Glo HD again in one shot, especially when I basically ran out of disk space, but would it be possible for the plugin to release all of the temporary disk space it's using when it's finished copying files to the device?