dashdemux: restarting not-linked stream is broken

Submitted by Thiago Sousa Santos @thiagossantos

Link to original bug (#739798)

Description

When a stream is not-linked dashdemux interrupts downloading it until a reconfigure event is received.

As it interrupts it immediately, the downstream demux might be left with a partial fragment which is an inconsistent state. If the stream is restarted it should make the demuxer flush its pending data before accepting the new fragment.

Another option is to have dashdemux only stop on fragment boundaries.