sqldb+graph/db: source nodes table, queries and CRUD

In this commit, we add the `source_nodes` table. It points to entries in
the `nodes` table. This table will store one entry per protocol version
that we are announcing a node_announcement on.

With this commit, we can run the TestSourceNode unit test against our
SQL backends.

sqldb/sqlc/models.go

@@ -130,3 +130,7 @@ type NodeFeature struct {
 	NodeID     int64
 	FeatureBit int32
 }
+
+type SourceNode struct {
+	NodeID int64
+}