media-export: Make grace period configurable

While it helps not to run discover on half-downloaded files, it slows down the
server sync use-case and breaks several DLNA requirements.

It can turned off now using monitor-grace-timeout=0

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=694455