decodebin3: add a fallback function when upstream changes and flush seek

When we try to flush seek to playbin3 pipeline, multiqueue pre is ongoing
upstream changes with auto-plugging.

In this scenario, below symptoms are observed.
  * Seek event can't be delivered to from multiqueue pre to source part.
    Because a pad of this part is not linked during auto plugging.
  * flush-start/stop event could not be delivered to decoder and sink
    elements by pad not linked.

To avoid this symptom, we try to seek event to parsebin sink and make sure
flush-start and flush-stop are delivered to decoder and sink elements.
22 jobs for wip_upstream_changes_with_seek in 104 minutes and 15 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #317297
gst indent

00:00:16

passed #317296
manifest

00:00:18

 
  Build
passed #317298
build fedora x86_64

00:05:30

passed #317299
build nodebug fedora x86_64

00:03:01

 
  Test
passed #317300
check fedora

00:03:38

passed #317301
integration testsuites fedora

00:08:05

 
  Full Builds
passed #317313
build cerbero android universal

00:13:27

passed #317314
build cerbero cross win32

00:06:52

passed #317316
build cerbero cross win64

00:07:42

passed #317315
build cerbero fedora x86_64

00:08:40

failed #317303
build cerbero android universal

00:01:08

failed #317304
build cerbero cross win32

00:01:04

failed #317305
build cerbero cross win64

00:01:04

failed #317302
build cerbero fedora x86_64

00:01:02

 
  Full Tests
passed #317309
valgrind bad

00:24:44

passed #319067
valgrind base

00:52:09

passed #317310
valgrind ges

00:05:03

passed #317307
valgrind good

00:07:37

passed #317308
valgrind ugly

00:02:12

failed #317306
valgrind base

00:54:25

 
  Apps
canceled #317311
android universal examples

 
  Deploy
canceled #317312
documentation