Network partition may lead to dataloss

Production cluster scenario that lead to data loss:

• 4 node cluster running on CB 2.5.1.
• node A, node B, node C failed-over node-D but failover message didn't reach node-D and it doesn't know it was failed over.
• SDK clients unfortunately were connecting to node-D for the vb-map config(which is incorrect).
• At this stage a set of vbuckets are active in 2 different places and once rebalance is initiated mutations mapping to active vbuckets on node-D are very likely going to get lost.

Sample data from logs to back it up:

World according to 10.100.0.46(node-D, which was failed over):

```
{incoming_replications_conf_hashes,
[{"services.z3",
[

{'ns_1@10.100.0.41',115455469},
{'ns_1@10.100.0.42',74343578},
{'ns_1@10.100.0.43',66117516}]},
{"default",
[{'ns_1@10.100.0.41',115455469}

,

{'ns_1@10.100.0.42',74343578}

,

{'ns_1@10.100.0.43',66117516}

]},
{"indigo-session",
[

{'ns_1@10.100.0.41',124324038},
{'ns_1@10.100.0.42',42364148},
{'ns_1@10.100.0.43',122578952}]},
{"services.z2",
[{'ns_1@10.100.0.41',124324038}

,

{'ns_1@10.100.0.42',42364148},
{'ns_1@10.100.0.43',122578952}]},
{"indigo",
[{'ns_1@10.100.0.41',48623555},
{'ns_1@10.100.0.42',124811468},
{'ns_1@10.100.0.43',104172491}]},
{"services",
[{'ns_1@10.100.0.41',124324038},
{'ns_1@10.100.0.42',42364148}

,

{'ns_1@10.100.0.43',122578952}

]}]},
```

World according to 10.100.0.42(is similar for node-A, node-B and node-C)

```
{incoming_replications_conf_hashes,
[{"services.z3",
[

{'ns_1@10.100.0.41',63506121},{'ns_1@10.100.0.43',117606290}]},
{"default",
[{'ns_1@10.100.0.41',63506121}

,

{'ns_1@10.100.0.43',117606290}

]},
{"indigo-session",
[

{'ns_1@10.100.0.41',101019889},{'ns_1@10.100.0.43',114028661}]},
{"services.z2",
[{'ns_1@10.100.0.41',101019889}

,

{'ns_1@10.100.0.43',114028661}]},
{"indigo",
[{'ns_1@10.100.0.41',51937030},{'ns_1@10.100.0.43',131953060}]},
{"services",
[{'ns_1@10.100.0.41',101019889},{'ns_1@10.100.0.43',114028661}

]}]},
```