cluster/afr: perform list-xattr during lookup

Detect and heal mismatching user extended attributes during lookup.

Depends on: http://review.gluster.org/#/c/7434/

Change-Id: I49410aafd319ac159fdf9e6f9201871bbf2f67bd
BUG: 1078061
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/7444
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>