Following variables do not require LOCK_open protection anymore:
- table_def_cache (renamed to tdc_hash) is protected by rw-lock
  LOCK_tdc_hash;
- table_def_shutdown_in_progress doesn't need LOCK_open protection;
- last_table_id use atomics;
- TABLE_SHARE::ref_count (renamed to TABLE_SHARE::tdc.ref_count)
  is protected by TABLE_SHARE::tdc.LOCK_table_share;
- TABLE_SHARE::next, ::prev (renamed to tdc.next and tdc.prev),
  oldest_unused_share, end_of_unused_share are protected by
  LOCK_unused_shares;
- TABLE_SHARE::m_flush_tickets (renamed to tdc.m_flush_tickets)
  is protected by TABLE_SHARE::tdc.LOCK_table_share;
- refresh_version (renamed to tdc_version) use atomics.