cluster/afr: Perform metadata sync inside metadata locks

Change-Id: Ie55b3d37cacbdad74a3f63c3b0f025b0ffd0104a
BUG: 1132461
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8514
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>