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.

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/slave.h b/sql/slave.h
index c6b78b96aca..cfff79da0e4 100644
--- a/sql/slave.h
+++ b/sql/slave.h
@@ -213,6 +213,7 @@ bool rpl_master_erroneous_autoinc(THD* thd);
 const char *print_slave_db_safe(const char *db);
 void skip_load_data_infile(NET* net);
 
+void slave_prepare_for_shutdown();
 void end_slave(); /* release slave threads */
 void close_active_mi(); /* clean up slave threads data */
 void clear_until_condition(Relay_log_info* rli);