Problem:
========
- During shutdown, InnoDB tries to free the asynchronous
I/O slots and hangs. The reason is that InnoDB disables
asynchronous I/O before waiting for pending
asynchronous I/O to finish.

buf_load(): InnoDB aborts the buffer pool load due to
user requested shutdown and doesn't wait for the asynchronous
read to get completed. This could lead to debug assertion
in buf_flush_buffer_pool() during shutdown

Fix:
===
os_aio_free(): Should wait all read_slots and write_slots
to finish before disabling the aio.

buf_load(): Should wait for pending read request to complete
even though it was aborted.