MDEV-4702 - Reduce usage of LOCK_open

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.

mysql-test/r/mysqld--help.result

@@ -927,6 +927,9 @@ The following options may be given as the first argument:
  The number of cached table definitions
  --table-open-cache=# 
  The number of cached open tables
+ --table-open-cache-instances=# 
+ MySQL 5.6 compatible option. Not used or needed in
+ MariaDB
  --tc-heuristic-recover=name 
  Decision to use in heuristic recover process. Possible
  values are COMMIT or ROLLBACK.
@@ -1258,6 +1261,7 @@ sysdate-is-now FALSE
 table-cache 400
 table-definition-cache 400
 table-open-cache 400
+table-open-cache-instances 1
 tc-heuristic-recover COMMIT
 thread-cache-size 0
 thread-pool-idle-timeout 60