On shutdown, do not exit threads in os_event_wait(). This method of
exiting was only used by the I/O handler threads. Exit them on a
higher level.

os_event_wait_low(), os_event_wait_time_low(): Do not exit on shutdown.

os_thread_exit(), ut_dbg_assertion_failed(), ut_print_timestamp(): Add
attribute cold, so that GCC knows that these functions are rarely
invoked and can be optimized for size.

os_aio_linux_collect(): Return on shutdown.

os_aio_linux_handle(), os_aio_simulated_handle(), os_aio_windows_handle():
Set *message1 = *message2 = NULL and return TRUE on shutdown.

fil_aio_wait(): Return on shutdown.

logs_empty_and_mark_files_at_shutdown(): Even in very fast shutdown
(innodb_fast_shutdown=2), allow the background threads to exit, but
skip the flushing and log checkpointing.

innobase_shutdown_for_mysql(): Always wait for all the threads to exit.

rb:633 approved by Sunny Bains