When the constant OS_AIO_N_PENDING_IOS_PER_THREAD is changed from 256 to 1
and the server is run with the minimum parameters
innodb_read_io_threads=1 and innodb_write_io_threads=2, two hangs
were observed.

tpool::cache<T>::put(T*): Ensure that get() in io_slots::acquire()
will be woken up when the cache previously was empty.

buf_pool_t::io_buf_t::reserve(): Schedule a possibly partial doublewrite
batch so that os_aio_wait_until_no_pending_writes() has a chance of
returning. Add a Boolean parameter and pass wait_for_reads=false inside
buf_page_decrypt_after_read(), because those calls will be executed
inside a read completion callback, and therefore
os_aio_wait_until_no_pending_reads() would block indefinitely.