- split part of ndb_multi.test which test drop database in combination
   with "stale" .ndb files in the MySQL Servers data dir.
 - support for such functionality will either be implemented as upgrade
   code or in the drop database code(although it hasn't been possible to
   add it there earlier)
 - disable new testcase
 - add comments in code describing the case with "stale" .ndb files
 - mark ndbcluster_drop_database_impl as static