We add a new NewV2Node constructor which takes a new NodeV2Fields as a
parameter. This NodeV2Fields struct defines the fields that can be set
in a models.Node if the version is V2.
The PubKey method had a race condition where concurrent calls could
all pass the nil check and race to write to the cached pubKey field.
This is a classic check-then-act race.
Remove the caching entirely to fix the race. The overhead of parsing
a public key is minimal and doesn't justify the added complexity and
race risk of caching.
In preparation for adding a NodeAnnouncement2 struct along with a
NodeAnnouncement interface, this commit renames the existing
NodeAnnouncment struct to NodeAnnouncement1.
