Almost all threads have gone
- the "ticking" threads, that sleep a while then do some work)
(srv_monitor_thread, srv_error_monitor_thread, srv_master_thread)
were replaced with timers. Some timers are periodic,
e.g the "master" timer.
- The btr_defragment_thread is also replaced by a timer , which
reschedules it self when current defragment "item" needs throttling
- the buf_resize_thread and buf_dump_threads are substitutes with tasks
Ditto with page cleaner workers.
- purge workers threads are not tasks as well, and purge cleaner
coordinator is a combination of a task and timer.
- All AIO is outsourced to tpool, Innodb just calls thread_pool::submit_io()
and provides the callback.
- The srv_slot_t was removed, and innodb_debug_sync used in purge
is currently not working, and needs reimplementation.
