Also had to rename the existing DEBUG consts to DEVMODE, so as to not
confuse the two. DEVMODE is meant for developers and cannot be enabled
on-the-fly, whereas debug mode is for users to check that the bridge is
working, MQTT messages are received etc.
Update logging is throttled to avoid spurious messages for uncoalesced
MQTT updates and motion sensors. On my network with 10 devices, an
update is logged every 1-2 minutes on average.
