When a node does not have routes, treat it like a stream and use
the restore-stream logic to restore the volumes.
Rework some of the logic a little. Don't save empty strings. Wait
for param updates to save/restore values.
This makes volume restore work on virtual sinks/sources and
sinks/sources without any routes.
See #729
The session manager might be configured without metadata support
so check this before attempting to use the metadata. It's not
really a problem because the default sink/source can only be
changed with the pulseaudio API, which activates metadata.
The prefix was used to filter the properties when loading and
saving but since the conversion to the core provided functions, this
feature is not longer available.
It's not exactly a problem, we could implement the filtering ourselves
afterwards but there is little point because we just ignore the unknown
items and never write invalid items.
Save settings as jason and use a small json compatible tokenizer to
load settings instead of our own less flexible format.
Save settings with a prefix and filter out entries without prefix
Listen for changes in restore.stream metadata and update properties.
Keep a per node entry of the saved target node and use this when
there is no other target node.
When explicitly moving a stream, save the new target node.
