cluster/afr: Don't start heal when lookup succeeds on < 2 children

Problem:
When self-heal code doesn't see at least 2 successes on looking up children,
then self-heal can't be done. What is happening now is if all the lookups fail
then the pending changelog is all zeros in xattrs so all the children are
becoming sources and leading to crashes when the code paths further assume that
some data structures are populated properly

Fix:
Don't proceed with self-heals when < 2 children succeed lookups.

BUG: 1128721
Change-Id: Iffdf0feebb6f98812d9d01cdd0cf97f3e19ba76f
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8698
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>