glusterd: Prevent rebalance starting with old clients

Glusterd will prevent rebalance from starting when clients older than
glusterfs-v3.6.0 are connected to a volume. This is needed as running
rebalance with old clients connected could lead to data loss in some
cases. The DHT xlator on newer clients (>= 3.6.0) has been fixed to
prevent the data loss issues.

Change-Id: If58640236382a2fc13f73f6b43777f01713859f7
BUG: 1136201
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/8583
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>