store: Fix resource leaks in gf_store_iter_* functions

Also, removed (redundant) member fd from gf_store_iter_t

Change-Id: I40f0469997f77fa2f578a5495ca4ce285f1a59f2
BUG: 904065
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/5243
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>