Adieu slave_list

slave_list was used to provide data for SHOW SLAVE HOSTS and
Slaves_connected status variable.

Introduced binlog_dump_thread_count which is exposed via Slaves_connected
(replaces slave_list.records).

Store Slave_info on THD and access it by iterating server_threads
(replaces slave_list).

Added:
THD::slave_info
binlog_dump_thread_count
show_slave_hosts_callback()

Removed:
slave_list
SLAVE_LIST_CHUNK
SLAVE_ERRMSG_SIZE
slave_list_key()
slave_info_free()
init_slave_list()
end_slave_list()
all_slave_list_mutexes
init_all_slave_list_mutexes()
key_LOCK_slave_list
LOCK_slave_list

Moved:
SLAVE_INFO -> Slave_info
register_slave() -> THD::register_slave()
unregister_slave() -> THD::unregister_slave()

Also removed redundant end_slave() from close_connections(): it is called
again soon afterwards by clean_up().

Pre-requisite for clean MDEV-18450 solution.

sql/sql_class.cc

@@ -642,6 +642,10 @@ THD::THD(my_thread_id id, bool is_wsrep_applier, bool skip_global_sys_var_lock)
    tdc_hash_pins(0),
    xid_hash_pins(0),
    m_tmp_tables_locked(false)
+#ifdef HAVE_REPLICATION
+   ,
+   slave_info(0)
+#endif
 #ifdef WITH_WSREP
    ,
    wsrep_applier(is_wsrep_applier),
@@ -1562,6 +1566,9 @@ void THD::cleanup(void)
   DBUG_ASSERT(!mdl_context.has_locks());
 
   apc_target.destroy();
+#ifdef HAVE_REPLICATION
+  unregister_slave();
+#endif
   cleanup_done=1;
   DBUG_VOID_RETURN;
 }