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.

graph/db/sql_store.go

@@ -55,6 +55,7 @@ type SQLQueries interface {
 	*/
 	UpsertNode(ctx context.Context, arg sqlc.UpsertNodeParams) (int64, error)
 	GetNodeByPubKey(ctx context.Context, arg sqlc.GetNodeByPubKeyParams) (sqlc.GraphNode, error)
+	GetNodesByIDs(ctx context.Context, ids []int64) ([]sqlc.GraphNode, error)
 	GetNodeIDByPubKey(ctx context.Context, arg sqlc.GetNodeIDByPubKeyParams) (int64, error)
 	GetNodesByLastUpdateRange(ctx context.Context, arg sqlc.GetNodesByLastUpdateRangeParams) ([]sqlc.GraphNode, error)
 	ListNodesPaginated(ctx context.Context, arg sqlc.ListNodesPaginatedParams) ([]sqlc.GraphNode, error)