gluster: Convert aio routines into coroutines Convert the read, write, flush and discard implementations from aio-based ones to coroutine based ones. Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>