## 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
c11253a82f