devicestate: Don't publish redundant device state messages.

devicestate: Don't publish redundant device state messages.

When publishing device state check the local cache for the
existing device state. If the new device state is unchanged
from the prior one, don't bother publishing the update. This
can reduce the work done by consumers of device state, such
as hints and app_queue, by not publishing a message to them.

These messages would most often occur with devices that are
seeing numerous simultaneous channels. The underlying device
state would remain as in use throughout, but an update would
be published as channels are created and hung up.