os_aio_wait_until_no_pending_reads(), os_aio_wait_until_pending_writes():
Add a Boolean parameter to indicate whether the wait should be declared
in the thread pool.

buf_flush_wait(): The callers have already declared a wait, so let us
avoid doing that again, just call os_aio_wait_until_pending_writes(false).

buf_flush_wait_flushed(): Do not declare a wait in the rare case that
the buf_flush_page_cleaner thread has been shut down already.

buf_flush_page_cleaner(), buf_flush_buffer_pool(): In the code that runs
during shutdown, do not declare waits.

buf_flush_buffer_pool(): Remove a debug assertion that might fail.
What really matters here is buf_pool.flush_list.count==0.

buf_read_recv_pages(), srv_prepare_to_delete_redo_log_file():
Do not declare waits during InnoDB startup.