discovery: make sure ChannelUpdates always have incremented timestamp
discovery: make sure ChannelUpdates always have incremented timestamp

This commit ensures that we always increment the timestamp of
ChannelUpdates we send telling the network about changes to
our channel policy. We do this because it could happen
(especially during tests) that we issued an update, but the
ChannelUpdate would have the same timestamp as our last
ChannelUpdate, and would be ignored by the network.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd