Route RawFileAsync data I/O through UringDataIo's owned-operation
retention path while keeping borrowed submissions available for the
legacy AsyncIo calls during the transition.
This mostly moves code around, temporarily moving uring handling from
RawFileAsync to the UringDataIo, including the unsafe iovec access.
This enables the UringIo to be added to RawFileAsync incrementally.
Later commits will remove the unsafe paths when the callers are updated
to use the new functions.
Signed-off-by: Dylan Reid <dgreid@fb.com>