aio: io_uring: Allow IO methods to check if IO completed in the background

aio: io_uring: Allow IO methods to check if IO completed in the background

Until now pgaio_wref_check_done() with io_method=io_uring would not detect if
IOs are known to have completed to the kernel, but the completion has not yet
been consumed by userspace.  This can lead to inferior performance and also
makes it harder to use smarter feedback logic in read_stream, because we
cannot use knowledge about whether an IO completed to control the readahead
distance.

This commit just adds the io_uring specific infrastructure. Later commits will
return whether a wait was needed from WaitReadBuffers() and then use that
knowledge.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
Discussion: https://postgr.es/m/CAH2-Wz%3DkMg3PNay96cHMT0LFwtxP-cQSRZTZzh1Cixxf8G%3Dzrw%40mail.gmail.com