cluster/afr: Perform metadata sync inside metadata locks

        Backport of http://review.gluster.org/8514

BUG: 1136825
Change-Id: I480a0dc0dfff8bcff084c9b3f048c5b355683f73
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8590
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>