Add LastStateChangeTime to chassis manager

This property is set to the timestamp of the last time
the chassis power state changed.  It is persisted
so it survives reboots.

Resolves openbmc/openbmc#3300

Tested:  Various incantations of power cycling, reboots,
         and AC pulls.

Change-Id: I19f244e0490bc9b921454e393989a9cbd283e2dd
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
3 files changed