cluster/dht - rebalance: handle the rebalance @ inode level (!fd level)

* migrate all the fd's on an inode to newer subvol after rebalance
* use the migration in progress flag in inode, so all the operations
  on the inode can make use of it

Change-Id: Ib807a46e927a1062688fc15119c916797c52a350
BUG: 1013456
Signed-off-by: Amar Tumballi <amarts@redhat.com>
Reviewed-on: http://review.gluster.org/5891
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>