graph/db+sqldb: use batch validation for node migration

Restructue the `migrateNodes` function so that it does the validation of
migrated nodes in batches. So instead of fetching each node individually
after migrating it, we wait for a minimum batch size to be reached and
then validate a batch of nodes together. This lets us make way fewer DB
round trips.

sqldb/sqlc/queries/graph.sql

@@ -21,6 +21,11 @@ WHERE graph_nodes.last_update IS NULL
     OR EXCLUDED.last_update > graph_nodes.last_update
 RETURNING id;
 
+-- name: GetNodesByIDs :many
+SELECT *
+FROM graph_nodes
+WHERE id IN (sqlc.slice('ids')/*SLICE:ids*/);
+
 -- name: GetNodeByPubKey :one
 SELECT *
 FROM graph_nodes