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>