composition: Do not remove all sources when stopping task

We only want to remove updates and seek, commits should be kept
source: Remove cruft code to seek sources

We now seek on ready and thus do not need to do magic trying to seek
the source as soon as possible as we now do it even sooner than soon.
composition: Rename _flush_downstream to _have_to_seek_downstrean
composition: Always return a value in GSourceFuncs
composition: set next_eos_seqnum when we get seeked too.