Merklized state pruning

[jbearer]

• When pruning up to height h, we can delete any node which
  • has created < h AND
  • is not the newest node with its pos
• In snapshot_info, where we currently check get_last_state_height() to check if the snapshot exists, we must also check pruned_height()
• There is no fetching for merklized state, so when rebuilding an archive (Config::archive), the application (i.e. sequencer) will need to ensure the update_state_storage_loop starts from 0
• Otherwise, the update_state_storage_loop should start from the maximum of get_last_state_height() and pruned_height() + 1 (we must start from an unpruned height so we can look up the parent state when applying the STF)