"Change in behavior --default-storage-engine=ndb or ndbcluster"
  Reduce use of legacy_db_type, some code cleanup
  (serg read my mind and implemented desired mysqld.cc changes)


sql/handler.cc:
  Bug#20168
    remove some use of legacy_db_type
    cleanup code, new func for default type
sql/handler.h:
  Bug#20168
    remove some use of legacy_db_type
    cleanup code, new func for default type
sql/sql_plugin.cc:
  compiler hints, consts
sql/sql_plugin.h:
  compiler hints, consts
sql/sql_tablespace.cc:
  use ha_default_handlerton instead of resolving DB_TYPE_DEFAULT