Prune old blocks from peer

Pruning old blocks from peer to alleviate concerns around long term storage growth.

The snapshot feature delivered in v2.3 enables organizations to compare ledgers on a channel and come to agreement around logical "checkpoints". These checkpointed snapshots can be saved off and used as a starting point for new peers so that they don't have to process all blocks since the genesis block. With the concept of snapshots and checkpoints established, it is feasible to prune blocks prior to the checkpointed ledger height.

This issue serves as a placeholder until a pruning RFC becomes available.