Events: fix signal duplication
Events: fix signal duplication

The same Device objects can be saved many times. Keep track of the last saved
status to avoid duplications.

Change-Id: Iec07264e7022e1b474f181c8f3acb691ecab69c4

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server