Make consensus validation optional for immutable

The consensus validation check (once enabled) will reduce the pipelining of blocks and is likely to slow bootstrapping from genesis down.  We only need to validate `BlockInfo::Immutable` blocks (from Mithril) if we're really paranoid.  Add a config option to Consensus to validate immutable blocks, default false.

```
[module.consensus]
validate-immutable = false
```


 