peerd: Add DiscoveredPeer

This object represents a peer we've discovered over one or more
technlogies.  It has logic to accept or reject updates to itself and
child services based on the timestamps and validity of the updates.  A
PeerManager may have many DiscoveredPeer instances.

BUG=chromium:418736
TEST=unittests

Change-Id: Iaa8e70c315fe096d6586c2ab2e2acaeda0ce7fe4
Reviewed-on: https://chromium-review.googlesource.com/222430
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
4 files changed