StagingBuffer:
We now allocate memory with mmap(MAP_ANONYMOUS) so that we can be
sure it can be pinned with io_uring_register_buffers(). This
ensures that if DMA is possible, it should be usable.
IoUringAssemblyEngine:
We now register an eventfd with io_uring so that we can listen
for CQEs with boost::asio.
hayodea