MB-100: batch_size=1 if len(self.buckets)>1 in _verify_all_buckets

I found that we often get Timeout error in items verification if we use async milti get verification with multiple buckets.
another way to fix this is to reduce the batch size from 1000 to 100
changes apply only _verify_all_buckets

Change-Id: Ia8d5ffe4643dce3b2f4bd969adfe9a3cd00d8577
Reviewed-on: http://review.couchbase.org/24959
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>
MB-100: track 'Broken pipe' in VBucketAwareMemcached

we have to track error: [Errno 32] Broken pipe at the end of the rebalance out and reset_vbuckets if it's so

http://qa.hq.northscale.net/job/centos-64-2.0-new-rebalance-mixed-cluster/50/consoleFull

Change-Id: Ibf5b889b9ac9007ad2df1d94d9a5d2909025fa18
Reviewed-on: http://review.couchbase.org/24960
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>