splitmuxsink: set the same seqnum on flush_start / flush_stop

It's currently not made mandatory by aggregator, but it might
eventually be, and is more consistent behaviour

See gstreamer/gst-plugins-bad#977
20 jobs for splitmuxsink-flush-muxer in 73 minutes and 57 seconds (queued for 1 minute and 13 seconds)
Status Job ID Name Coverage
  Preparation
passed #357807
gst indent

00:01:09

passed #357806
manifest

00:00:30

 
  Build
passed #357813
build cerbero android universal

00:13:19

passed #357814
build cerbero cross win32

00:05:32

passed #357815
build cerbero cross win64

00:09:32

passed #357812
build cerbero fedora x86_64

00:05:14

passed #357816
gst-macos-10.14
build cerbero macos x86_64

00:08:07

passed #357808
build fedora x86_64

00:07:15

passed #357809
build nodebug fedora x86_64

00:05:36

passed #357810
build static fedora x86_64

00:13:52

passed #357811
build static nodebug fedora x86_64

00:11:15

 
  Test
passed #358481
check fedora

00:09:24

passed #357818
integration testsuites fedora

00:08:48

passed #357820
valgrind bad

00:10:53

passed #357821
valgrind ges

00:03:58

passed #357819
valgrind good

00:10:40

failed #358080
check fedora

00:05:07

failed #357817
check fedora

00:05:38

 
  Integrate
passed #357822
android universal examples

00:06:26

passed #357823
documentation

00:04:13