glusterfs-server : barrier timeout tuning fix

Problem : Reconfiguration of barrier timeout through gluster volume set shows a
success but it never changes the default timeout value which is 120 seconds.
After digging into the code deeper, it was found that timeout is never modified
in reconfigure() as the first check i.e. whether barrier is already enabled or
disabled always fails since barrier option is not modified in this request.

Fix : Introduced notify() in barrier translator which will take care of the rpc
request to enable/disable barrier. reconfigure() will simply set barrier
enable/disable and timeout options blindly without any validation.

Please note this patch only contains the changes in barrier translator however
from complete code flow perspective the caller in the glusterfsd mgmt should
call notify instead of reconfigure to fix this problem.

Change-Id: I1371b294935f6054da7c1dc6a9a19f1d861e60fb
BUG: 1085671
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/7428
Reviewed-by: Varun Shastry <vshastry@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>