oggdemux: fix a race in push mode when performing the duration seek

There may be two or more threads involved here however the important
interaction is the use of ogg->seeK_event_drop_till value that was only
set in the push-mode seek-event thread and could race with upstream
sending e.g. and EOS (or data).

Scenario is this:
1. oggdemux performs a seek to near the end of the file to try and find
   the duration. ogg->push_state is set to PUSH_DURATION.
2. Seek is picked up by the dedicated seek event thread and sets
   ogg->seek_event_drop_till to the seek event's seqnum.
3. Most operations are blocked or dropped waiting on the duration to
   be determined and processing continues until a duration is found.
4. Two branching options for how this ultimately plays out
4a. The source is too fast and we receive an EOS event which is dropped
    because ogg->push_state == PUSH_DURATION.  In this case everything
    works.
4b. We hit our 'almost at the end' check in
    gst_ogg_pad_handle_push_mode_state() and attempt to seek back to the
    beginning (or to a user-provided seek).  This seek is marshalled to
    the seek event thread without setting ogg->seek_event_drop_till but
    with change ogg->push_state = PUSH_PLAYING.  If an EOS event or
    e.g. buffers arrive from upstream before the seek event thread has
    picked up the seek event, then the EOS/data is processed as if it
    came as a result of the seek event.  This is the case that fails.

The fix is two-fold:
1. Preemptively set ogg->seek_event_drop_till when setting the seek
   event so that data and other events can be dropped correctly.
2. In addition to dropping and EOS events while ogg->push_state ==
   PUSH_DURATION, also drop any EOS events that are received before the
   seek event has been processed by also tracking the seqnum of the seek.

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