Nodes referencing a device can be bound later than the device is
bound. If this happens, the node will not receive an initial route
device volume change event. This change caches the last known route
device volume and initializes the device with it if present.
Yet another device node edge case. In addition to only writing via
a pw_device when present, now we only read from one as well.
This fixes missing state changes not conveyed by the pw_node.
Fixes#35