cluster/afr: Launch self-heal only when all the brick status is known

Problem:
File goes into split-brain because of wrong erasing of xattrs.

RCA:
The issue happens because index self-heal is triggered even before all the
bricks are up. So what ends up happening while erasing the xattrs is, xattrs
are erased only on the sink brick for the brick that it thinks is up leading to
split-brain

Example:
lets say the xattrs before heal started are:
brick 2:
trusted.afr.vol1-client-2=0x000000020000000000000000
trusted.afr.vol1-client-3=0x000000020000000000000000

brick 3:
trusted.afr.vol1-client-2=0x000010040000000000000000
trusted.afr.vol1-client-3=0x000000000000000000000000

if only brick-2 came up at the time of triggering the self-heal only
'trusted.afr.vol1-client-2' is erased leading to the following xattrs:

brick 2:
trusted.afr.vol1-client-2=0x000000000000000000000000
trusted.afr.vol1-client-3=0x000000020000000000000000

brick 3:
trusted.afr.vol1-client-2=0x000010040000000000000000
trusted.afr.vol1-client-3=0x000000000000000000000000

So the file goes into split-brain.

Change-Id: I1185713c688e0f41fd32bf2a5953c505d17a3173
BUG: 1142601
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8755
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>