[FEATURE REQUEST] Specify a sqe won't generate a cqe

["Carter Li 李通洲" <carter.li@eoitek.com>]

To implement io_uring_wait_cqe_timeout, we introduce a magic number
called `LIBURING_UDATA_TIMEOUT`. The problem is that not only we
must make sure that users should never set sqe->user_data to
LIBURING_UDATA_TIMEOUT, but also introduce extra complexity to
filter out TIMEOUT cqes.

Former discussion: https://github.com/axboe/liburing/issues/53

I’m suggesting introducing a new SQE flag called IOSQE_IGNORE_CQE
to solve this problem.

For a sqe tagged with IOSQE_IGNORE_CQE flag, it won’t generate a cqe
on completion. So that IORING_OP_TIMEOUT can be filtered on kernel
side.

In addition, `IOSQE_IGNORE_CQE` can be used to save cq size.

For example `POLL_ADD(POLLIN)->READ/RECV` link chain, people usually
don’t care the result of `POLL_ADD` is ( since it will always be
POLLIN ), `IOSQE_IGNORE_CQE` can be set on `POLL_ADD` to save lots
of cq size.

Besides POLL_ADD, people usually don’t care the result of POLL_REMOVE
/TIMEOUT_REMOVE/ASYNC_CANCEL/CLOSE. These operations can also be tagged
with IOSQE_IGNORE_CQE.

Thoughts?