Nowadays, users should use the standard "PropertiesChanged" signal
on "org.freedesktop.DBus.Properties" interface.
(cherry picked from commit 6fb917178a)
Instead of updating the device-statistic counters only periodically as
we refresh the link, update them on every link-changed event from
platform.
That means, also for devices that have RefreshRateMs at zero, the values
will be updated at random times when the link information changes.
The difference is, that previously the counters would be zero unless
RefreshRateMs was set. Now, they have some (probably stale) values
which however are not guaranteed to be kept up-to-date.
Also, now we refresh more often then promised by RefreshRateMs. But the API
technically doesn't specify that, so if we find there is a problem with
this, we may revert it later.
Add statistics interface to all device instances. When active, the
properties of this interface are refreshed whenever there is network
activity for the device.
Activation is performed by changing RefreshRateMs property. If set to
zero, the interface is deactivated. If set to other value, the rest of
the interface properties are refreshed whenever the related network
metric changes, being RefreshRateMs the minimum time between property
changes, in milliseconds.
