media-db: Make Database.exec cancellable
media-db: Make Database.exec cancellable

Pelagicore/Media-Manager-Rygel-patched,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,luisbg/rygel,GNOME/rygel,Tarnyko/rygel,Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,jku/rygel