store: move glusterd_store functions from mgmt/glusterd to libglusterfs

Making the glusterd_store_* functions re-usable will help with future
changes that need to read/write lists of items.

BUG: 904065
Change-Id: I99fb8eced76d12d5a254567eccff9790b43d8da3
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/4676
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>