This implements a priority queue backed by the wallet database for scan range ordering. The scan queue is updated on each call to `put_blocks` or to `update_chain_tip`.
The lightwalletd service file is sourced from: Git: https://github.com/zcash/lightwalletd Rev: ad5ecda5fbb5e12799f926b8cd785d54fdd250c8 Closes zcash/librustzcash#585.