[IO] improve Future member types

Use int32_t as Future returnValue instead of ssize_t since
io_uring uses int32_t not ssize_t as results value therefore it is
pointless to use a bigger type.

This shrinks the future size and prevents useless and implementation defined
narrowing conversions from ssize_t to int32_t.

Use union for flags and offset.

Ignore int to pointer cast clang-tidy warning.