Adding this datastructure will allow us to resolve the
issues hashicorp#1088 and hashicorp#4198

This new structure defaults to values:
```
   { Passing: 1, Warning: 0 }
```

Which means, use weight of 0 for a Service in Warning State
while use Weight 1 for a Healthy Service.
Thus it remains compatible with previous Consul versions.

… (backwards compatibility)

When using DNS interface with only_passing = false, all nodes
with non-Critical healthcheck used to have a weight value of 1.
While having weight.Warning = 0 as default value, this is probably
a bad idea as it breaks ascending compatibility.

Thus, we put a default value of 1 to be consistent with existing behaviour.

…instances

Will allow master branch to be merged properly

…hts as requested by @banks