librados/asio: clear cancellation slot in associated executor

librados/asio: clear cancellation slot in associated executor

the librados callback function `AsyncOp::aio_dispatch()` runs on
Objecter's finisher strand executor, and dispatches the completion
handler to its associated executor

asio cancellation is not thread-safe, so should be synchronized on that
associated executor. move the call to `slot.clear()` from that librados
callback into the AsyncHandler wrapper so it doesn't run until we've
switched to the correct executor

because our `op_cancellation` handler depends on an `AioCompletion`
pointer, we have to clear the cancellation slot before that
`AioCompletion` lifetime ends

Fixes: https://tracker.ceph.com/issues/73475
Signed-off-by: Casey Bodley <cbodley@redhat.com>