matroskademux: Guard read state transitions with locks

This patch adds locks to all pieces of code that were changing the read
state of matroskademux and were not already locking it.

This is necessary for seek event deferring to work without races.
6 jobs for matroskademux-defer-seek in 29 minutes and 43 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #64284
gst indent

00:00:28

passed #64283
manifest

00:00:22

 
  Build
passed #64285
build fedora x86_64

00:02:40

 
  Test
passed #64286
check fedora

00:15:43

 
  Full Builds
passed #64287
build android arm64 api28

00:03:00

passed #64288
build cerbero fedora x86_64

00:10:50