db: fix sqlite3 code which manipulates columns.

Because it used internal routines, it didn't pass operations through the
db hook!  So make it use the generic routines, with the twist that they
are not translated.

And when we use this in a migration hook, we're actually in a
transaction.

This, in turn, introduces an issue: we need to be outside a transaction
to "PRAGMA foreign_keys = OFF", but completing the transaction when
there is a db hook actually enters the io loop, freeing the tmpctx!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

db/utils.h

@@ -97,4 +97,12 @@ void db_assert_no_outstanding_statements(struct db *db);
  */
 const char **db_changes(struct db *db);
 
+/**
+ * Accessor for internal use.
+ *
+ * Like db_prepare_v2() but creates temporary noop translation, and
+ * assumes not a read-only op.  Use this inside db-specific backends
+ * to re-use the normal db hook and replication logic.
+ */
+struct db_stmt *db_prepare_untranslated(struct db *db, const char *query);
 #endif /* LIGHTNING_DB_UTILS_H */