curr_items is not equal to actual # of keys inserted : extra docs during failover

build-1577

test http://qa.hq.northscale.net/job/ubuntu-64-2.0-rebalance-tests/50/

steps:
1.cluster 4 nodes, 3 buckets, upload data using mcsoda
2.during docs ops ( delete/add/expire) failover one node
result: 'curr_items : 54068 is not equal to actual # of keys inserted : 54003 : bucket: bucket-0'

it's possible to reproduce as separate test, for instance:
./testrunner -i FILE.ini -t rebalancetests.RebalanceOutWithFailover.test_load

execution result and diags are attached

Chiyoung, you can take these vms to review( cleanup was skipped ) or to reproduce:

[global]
username:jenkins
ssh_key:~/.ssh/QAkey.pem
port:8091

[membase]
rest_username:Administrator
rest_password:password

[servers]
1:vm1
2:vm2
3:vm3
4:vm4

[vm1]
ip:10.5.2.11

[vm2]
ip:10.5.2.13

[vm3]
ip:10.5.2.14

[vm4]
ip:10.5.2.15