This commit converts all the uses of wgkey to the new key interfaces.
It now has specific machine, node and discovery keys and we now should
use them correctly.
Please note the new logic which strips a key prefix (in utils.go) that
is now standard inside tailscale.
In theory we could put it in the database, but to preserve backwards
compatibility and not spend a lot of resources on accounting for both,
we just strip them.
