Once we merge state sync and block pruning, it would be nice to be able to backfill historical blocks and associated state metadata (e.g. validator sets and commit info). See RFC-001 for discussion.

For state sync, this would allow apps to enforce a minimum block history invariant via e.g. a backfill_height metadata field in state sync snapshots and a retain_height ABCI commit response, and for block pruning this would allow a node with a truncated history to be converted into an archival node.

The existing blockchain reactor should probably be extended to handle this.

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