Pruning feature

[unwriter]

It's ideal to keep the entire bitbus, but in some cases developers may not want to keep the bus state, or may only want to keep the most recent blocks.

Think about adding a feature that allows:

1. Pruning older blocks as a setting, so it only keeps the N most recent blocks
2. Make N configurable.

Will need to take into account the re-org API which will need to be implemented, because if the bus is completely gone it will be difficult to detect re-orgs locally.