MDEV-9573 'Stop slave' hangs on replication slave

The reason for this is that stop slave takes LOCK_active_mi over the
whole operation while some slave operations will also need LOCK_active_mi
which causes deadlocks.

Fixed by introducing object counting for Master_info and not taking
LOCK_active_mi over stop slave or even stop_all_slaves()

Another benefit of this approach is that it allows:
- Multiple threads can run SHOW SLAVE STATUS at the same time
- START/STOP/RESET/SLAVE STATUS on a slave will not block other slaves
- Simpler interface for handling get_master_info()
- Added some missing unlock of 'log_lock' in error condtions
- Moved rpl_parallel_inactivate_pool(&global_rpl_thread_pool) to end
  of stop_slave() to not have to use LOCK_active_mi inside
  terminate_slave_threads()
- Changed argument for remove_master_info() to Master_info, as we always
  have this available
- Fixed core dump when doing FLUSH TABLES WITH READ LOCK and parallel
  replication. Problem was that waiting for pause_for_ftwrl was not done
  when deleting rpt->current_owner after a force_abort.

sql/rpl_parallel.h

@@ -342,6 +342,7 @@ struct rpl_parallel {
 extern struct rpl_parallel_thread_pool global_rpl_thread_pool;
 
 
+extern int rpl_parallel_resize_pool_if_no_slaves(void);
 extern int rpl_parallel_activate_pool(rpl_parallel_thread_pool *pool);
 extern int rpl_parallel_inactivate_pool(rpl_parallel_thread_pool *pool);
 extern bool process_gtid_for_restart_pos(Relay_log_info *rli, rpl_gtid *gtid);