rpl.rpl_semi_sync_wait_point crashes because of thd_destructor_proxy

rpl.rpl_semi_sync_wait_point crashes because of thd_destructor_proxy kills innodb
service threads before all slave threads has ended.

What happens is that proxy detects that no transactions are active and starts
srv_shutdown_bg_undo_sources(), but fails to take into account that new transactions
can still start, especially be slave but also by other threads. In addition there is no
mute when checking for active transaction so this is not safe.

Suggestion is to mark innodb server threads and in close_connection first shutdown all other threads, including events, and then last inform destructor proxy and other innodb threads that they can now safely be shut down.