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.