matroskademux: Defer seeks received before GST_MATROSKA_READ_STATE_DATA

This patch enables matroskademux to receive seeks before it reaches
GST_MATROSKA_READ_STATE_DATA. This fixes the following regression:

gstreamer/gst-plugins-good#514

This also enables receiving seeks in the element READY state.

When such a seek is received, it is stored to be later handled when
GST_MATROSKA_READ_STATE_DATA is reached.
Status Job ID Name Coverage
  Manifest
passed #43879
manifest

00:00:21

 
  Build
failed #43925
build fedora x86_64

01:28:49

failed #43924
build fedora x86_64

00:02:13

failed #43880
build fedora x86_64

01:26:39

 
Name Stage Failure
failed
build fedora x86_64 Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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