If the default sink or source is set as the target, just don't save
the new target so that it follows the new default.
This is a workaround for gnome-control-center first setting the
default source/sink and then setting the targets on all
stream-restore database entries to the new default. Setting an
explicit target will pin the stream to that sink/source forever,
which is then making pavucontrol default source/sink toggle
fail.
See #676
Convert to an from pulse enum to id in the message layer so that we
can always just deal with native spa types.
Use the channelmap in stream-restore
Parse the channelmap from properties.
