graph/db+sqldb: use batch validation for closed SCID migration

As was done in the previous commits for nodes & channels, we update the
migrateClosedSCIDIndex function here so that it validates migrated
entries in batches rather than one-by-one.

sqldb/sqlc/queries/graph.sql

@@ -984,3 +984,8 @@ SELECT EXISTS (
     FROM graph_closed_scids
     WHERE scid = $1
 );
+
+-- name: GetClosedChannelsSCIDs :many
+SELECT scid
+FROM graph_closed_scids
+WHERE scid IN (sqlc.slice('scids')/*SLICE:scids*/);