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.

Closes 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 #43988
manifest

00:00:21

 
  Build
passed #44229
build fedora x86_64

00:01:26

failed #43989
build fedora x86_64

00:01:40