- The mysqld participating in schema distribution
   of DROP DATABASE failed to remove the database
   when there was a .ndb file in the directory
   which didn't have any corresponding table in NDB.
 - The problem is generic and really not related to schema distribution
   participant, it can be reproduced with only one mysqld
 - Fix by adding an additional check for leftover .ndb
   files in ndbcluster's DROP DATABASE implementation. The
   fix is a rewrite and thus intentionally mimics the beaviour
   of find_db_tables_and_rm_known_files() which previously
   deleted these files.
 - Add testcase reproducing with one mysqld
 - Add testcase reproducing with two mysql(s), same test as for
   bug44529 but with comments.