gfapi: async discard support

From: Bharata B Rao <bharata@linux.vnet.ibm.com>

Add glfs_discard_async() to meet QEMU's discard requirements.

BUG: 963678
Change-Id: I97723107fca88dd96cddc08cd67a286645a37922
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/5144
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>