Check difficulty for reconcile blocks

With nodes behind a load balancer the polling algorithm may return different last blocks may start bouncing between two different headers. For example:

```
...
t: Add: 0x1, Remove: 0x2
t + 1: Add: 0x2, Remove: 0x1
t + 2: Add: 0x1, Remove: 0x2
...
```

One solution would be to check the difficulty of the new headers before reconciling.