tagdemux: Forward TIME seeks upstream too, maybe upstream can handle that

https://bugzilla.gnome.org/show_bug.cgi?id=723597
rtspconnection: Call closed() when GET is closed in tunneled mode

This patch adds read source on the write socket in tunneled
mode and we get a callback when client disconnects the GET
channel.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=725313