Previously a hardcoded 0.0001 offset was used to determine if a volume
change was significant enough to send to a device, however some
devices have a much more granular step size, which caused future
volume updates to be blocked.
This change replaces the hardcoded offset with the volumeStep device
route property which should be large enough for the device to work with.
Fixes#279
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
