Adjust default offline threshold
Adjust default offline threshold

```
z = 0.9999 // target percentage
y = 5 // number of nodes
z = 0.841510681 // node percentage
--------
1 - ((1 - x) ^ y) = z
--------
1 = z + ((1 - x) ^ y)
--------
1 - z = (1 - x) ^ y
--------
(1 - z) ^ 1/y = 1 - x
--------
1 - ((1 - z) ^ 1/y) = x
--------
```

Meeting notes:
https://github.com/Storj/dev-meetings/blob/master/2017-12-06-summary.md

Storj/bridge,Storj/bridge,Storj/metadisk-api,Storj/metadisk-api