Check block ancestor is in chain before propagation

## Description

Check the `finalized_checkpoint` is an ancestor of a gossiped block before gossiping. 

Specifically this from the network specs:

```
The current finalized_checkpoint is an ancestor of block -- i.e. get_ancestor(store, block.parent_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)) == store.finalized_checkpoint.root 
```
