basesrc: preserve seqnum on segments after seeks

basesrc: preserve seqnum on segments after seeks

The seqnum of the segment after a seek should be the same of
the seek event. Downstream elements might rely on seqnums to
identify events related to a seek.

This is particularly important when a demuxer maps a TIME seek
into a BYTES seek for upstream and it needs to identify the
corresponding segment event and map it back into TIME to push
downstream, possibly using the values from the original seek
event.

https://bugzilla.gnome.org/show_bug.cgi?id=707530