composition: Post messages on the bus when it updates itself

And properly set the seqnums of those messages so that the application,
parents have the exact information about what is going on and why.
composition: Do not try to paused the task that could have been stopped

There was a race where we ended up trying to update the pipeline and
stop our children task at the exact moment where we were actually
setting its state to PAUSED. Take the composition lock and make sure
that can't happen
composition: Also ignore all messages from children tearing them to READY

At that stage elements should not be taken into account anymore. In some
spacial cases they can post ERROR messages (when trying to push sticky
events on flushing pads) on the bus. We actually do not care about those
issues at that exact point.