cluster/afr: Fix spurious metadata self-heals

- Added logging for metadata and data self-heals which helped
  in debugging this issue.
- Added checks to skip self-heals when no sinks are available to heal

Change-Id: I0d50dceb84cd9ad4fe00e0b749ddf7d4ff42348a
BUG: 1128721
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8709
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>