Fix local mesh gateway with peering discovery chains.
Prior to this patch, discovery chains with peers would not
properly honor the mesh gateway mode for two reasons.
1. An incorrect target upstream ID was used to lookup the
mesh gateway mode. To fix this, the parent upstream uid is
now used instead of the discovery-chain-target-uid to find
the intended mesh gateway mode.
2. The watch for local mesh gateways was never initialized
for discovery chains. To fix this, the discovery chains are
now scanned, and a local GW watch is spawned if: the mesh
gateway mode is local and the target is a peering connection.
Derek Menteer