core: fixed chain reorg during splits

Chain reorgs weren't properly handled when a chain was further ahead.
Previously we'd end up with mixed chains in our canonical numbering
sequence. Added test for this type of forking.

```
     /-o-o-o   A
o-C-+
     \-o-o-o-o B
```

Ends up with with C A1, A2, A3, B4