Fixed MDEV-306 / LP:1007967 - Assertion `table->file->stats.records > 0 || error' failed join_read_const_table on concurrent SELECT and DROP/ADD INDEX


sql/sql_table.cc:
  Added comment
storage/maria/ma_close.c:
  Don't store history if it's visible to all.
  This fixed the MDEV-306 bug
storage/maria/ma_delete_table.c:
  Removed old comment
  Delete history state for deleted tables
storage/maria/ma_info.c:
  More DBUG_PRINT
storage/maria/ma_open.c:
  More DBUG_PRINT