sqldb+graph/db: implement SQLStore.NodeUpdatesInHorizon

In this commit we add the necessary SQL queries and then implement the
SQLStore's NodeUpdatesInHorizon method. This lets us run the
TestNodeUpdatesInHorizon unit tests against SQL backends.

sqldb/sqlc/queries/graph.sql

@@ -82,6 +82,12 @@ LEFT JOIN node_addresses a ON a.node_id = n.id
 WHERE n.pub_key = $1 AND n.version = $2
 ORDER BY a.type ASC, a.position ASC;
 
+-- name: GetNodesByLastUpdateRange :many
+SELECT *
+FROM nodes
+WHERE last_update >= @start_time
+  AND last_update < @end_time;
+
 -- name: DeleteNodeAddresses :exec
 DELETE FROM node_addresses
 WHERE node_id = $1;