Remove grandparents from snapshot cache (#2917)

## Issue Addressed

NA

## Proposed Changes

In https://github.com/sigp/lighthouse/pull/2832 we made some changes to the `SnapshotCache` to help deal with the one-block reorgs seen on mainnet (and testnets).

I believe the change in #2832 is good and we should keep it, but I think that in its present form it is causing the `SnapshotCache` to hold onto states that it doesn't need anymore. For example, a skip slot will result in one more `BeaconSnapshot` being stored in the cache.

This PR adds a new type of pruning that happens after a block is inserted to the cache. We will remove any snapshot from the cache that is a *grandparent* of the block being imported. Since we know the grandparent has two valid blocks built atop it, it is not at risk from a one-block re-org. 

## Additional Info

NA

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -2796,6 +2796,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                         beacon_block_root: block_root,
                     },
                     None,
+                    &self.spec,
                 )
             })
             .unwrap_or_else(|e| {
@@ -3740,6 +3741,12 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             .try_write_for(BLOCK_PROCESSING_CACHE_LOCK_TIMEOUT)
             .map(|mut snapshot_cache| {
                 snapshot_cache.prune(new_finalized_checkpoint.epoch);
+                debug!(
+                    self.log,
+                    "Snapshot cache pruned";
+                    "new_len" => snapshot_cache.len(),
+                    "remaining_roots" => ?snapshot_cache.beacon_block_roots(),
+                );
             })
             .unwrap_or_else(|| {
                 error!(