MDEV-5289: master server starts slave parallel threads

Delay spawning parallel replication worker threads until a slave SQL
thread is running, and de-spawn them when the last SQL thread stops.

This is especially useful to avoid needless threads on a master in a
setup where same my.cnf is used on masters and slaves.

sql/rpl_mi.h

@@ -218,6 +218,7 @@ public:
   Master_info *get_master_info(LEX_STRING *connection_name,
                                Sql_condition::enum_warning_level warning);
   bool give_error_if_slave_running();
+  bool any_slave_sql_running();
   bool start_all_slaves(THD *thd);
   bool stop_all_slaves(THD *thd);
 };