discovery: Add timestamp to discovery.Update.

The syncbase discovery service needs to be able to order updates from
global discovery and neighborhood discovery based on timestamps in the
case of a changed advertisement.
This change exposes timestamps from discovery.Update.

MultiPart: 5/5
Change-Id: Ic2d22c1c1090a8521d7964e5e4f0b3b24f9000e7
2 files changed