server: update genNodeAnnouncement to also write new node ann to disk
server: update genNodeAnnouncement to also write new node ann to disk

In this commit, we update the genNodeAnnouncement method to also write
an updated version of the node announcment to disk. Before this commit,
we would update the in memory version, but then never write the new
version to disk. As a result, when connecting to new peers, we would
never propagate the new version of this announcement to the network.

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