Attempt to fix Link vanishing after mastership change

- LLDPLinkProvider: handle DeviceUpdate event
- DeviceManager: publish Device events caused by Mastership change
- DeviceManager: Always try to markOffLine on deviceDisconnected
- GossipDeviceStore: Silently ignore failure to get Timestamp on port events

Change-Id: I51fbb3f1924007867512f20e62d6d53090c63640
5 files changed