decodebin3: Handle dual-output of STREAM_START/EOS

In order to flush out multiqueue, we send again a STREAM_START and
then a EOS event.

The problem was that was that we might end up pushing out on the
output of multiqueue (and therefore decodebin3) a series of:
* EOS / STREAM_START / EOS

Apart from the uglyness of such output, If decodebin3 is used with
elements such as concat on their output, they might potentially
block on that second STREAM_START.

In order to make sure we don't end up in that situation we send
a custom STREAM_START event when refreshing multiqueue (which we
drop on the output) and we don't special case EOS events on streams
on which we already got EOS.

At worst we now end up sending at most two EOS on the output of
multiqueue (and decodebin3).